Maxime Besacier

Shadowing effect minimization in EUV mask by modeling

In Extreme Ultraviolet Lithography, the electromagnetic modeling of the mask allows to determine the influence of the mask structure on the electromagnetic field and on the aerial image. It is very useful to study the effect of the shape absorber on the CD shift. This effect, called shadowing effect, is analyzed in this paper. A simple geometrical approach to address this phenomenon is presented first. It is shown that although it can qualitatively be drawn some first orders conclusions, this over simplified view is unable to explain the complex behavior of the reflected light field. A rigorous method is still the more adapted method to assess the influence of the geometrical parameters of the features on the mask to control the CD shift on the printed resist. This study is especially focused on the absorber edges slope. It is demonstrated the choice of edge angles can minimize CD shift or keep a constant CD width.

Simulation of extreme ultraviolet masks with defective multilayers

Because the capabilities for experimental studies are still limited, a predictive simulation of EUV lithography is very important for a better understanding of the technology. One of the most critical issues in EUV lithography modeling is the description of the mask, especially including multilayer defects. A new model for the characterization of defects in the multilayer of an EUV reflective mask is presented. The mask is divided into an absorber part, which defines the features on the mask, and a multilayer part, which determines the reflectivity of the mask without absorber. Since the height of the mask features is large in comparison to the illumination wavelength, the computation of the absorber part is performed by a finite-difference time-domain (FDTD) method. Because of the limited range of illumination angles with a high reflectivity and the limited diffraction efficiency of the multilayer, the computation of the reflectivity of the defective multilayer is performed by the Fresnel-method. The defect topography is taken into account by means of correcting the phase and the angle of incidence. For the complete computation of the reflected light from the EUV mask a coupling of the two methods is realized. Thus, the model can be applied to two and three dimensional defects and masks. The impact of the defects on the mask reflectivity, the near field and the aerial image is analyzed. Typical mask structures, such as 2D-lines and 3D-dots with various defects, are investigated. First comparisons with another simulation model, the MMFE method, are presented.

Advanced OPC Mask-3D and Resist-3D modeling

The objective of this paper is to extend the ability of a more stable overall process control for the 28 nm Metal layer. A method to better control complex 2D-layout structures for this node is described. Challenges are coming from the fact that the structures, which limit the process window are mainly of 2D routing nature and are difficult to monitor. Within the framework of this study the emphasis is on how to predict these process-window-limiting structures upfront, to identify root causes and to assist in easier monitoring solutions enhancing the process control. To address those challenges, the first step is the construction of a reliable Mask-3D and Resist-3D model. Advanced 3Dmodeling allows better prediction of process variation upfront. Furthermore it allows highlighting critical structures impacted by either best-focus shifts or by low-contrast resist-imaging effects, which then will be transferred non-linearly after etch. This paper has a tight attention on measuring the 3D nature of the resist profiles by multiple experimental techniques such as Cross-section scanning electron microscopy methods (X-SEM) and atomic force microscopy (AFM). Based on these measurements the most reliable data are selected to calibrate full-chip Resist-3D model with. Current results show efficient profile matching among the calibrated R3D model, wafer AFM and X-SEM measurements. In parallel this study enables the application of a new metric as result of the resist profiles behavior in function of exposure dose. In addition it renders the importance on the resist shape. Together these items are reflected to be efficient support on process optimization and improvement on the process control.

Real-time profile shape reconstruction using dynamic scatterometry

In-line process control in microelectronics manufacturing requires real-time and non-invasive monitoring techniques. Among the different metrology techniques, scatterometry, based on the analysis of ellipsometric signatures (i.e stokes coefficients vs. wavelength) of the light scattered by a patterned structures, seems to be well adapted. Traditionally, the problem of defining the shape and computing the signature is dealt with modal methods and is called direct problem. On the opposite, the inverse problem allows to find the grating shape thanks to an experimental signature acquisition, and can not be solved as easily. Different classes of algorithms have been introduced (evolutionary, simplex, etc.) to address this problem, but the method of library searching seems to be the most attractive technique for industry. This technique has many advantages that will be presented in this article, however the main limitation in real-time context comes from the short data acquisition time for different wavelengths. Indeed, the lack of data leads to the method failure and several database patterns can match the experimental data. In this article, a technique for real time reconstruction of grating shape variation using dynamic scatterometry is presented. The different tools to realize this reconstruction, such as Modal Method by Fourier Expansion, regularization technique and specific software and hardware architectures are then introduced. Results issued from dynamic experiments will finally illustrate this paper.

Three-dimensional rigorous simulation of EUV defective masks using modal method by Fourier expansion

In Extreme Ultraviolet Lithography, the electromagnetic modelling of the mask allows determining the influence of the mask structure on the electromagnetic field. That makes it possible to take into account the presence of a defect modifying the multi-layer stack1,2. The method used throughout this paper is the MMFE (Modal Method by Fourier Expansion) also known as the RCWA (Rigorous Coupled Wave Analysis). Modal methods allow computing the electromagnetic field just above the EUV mask or the near field. Modal methods are well adapted for EUV mask simulation due to materials and structure size. The previous works performed on 2D simulation with MMFE3 have shown the influence of a defect inside a EUV mask structure. In this article, the method is extended to address 3D structures. The printability of a spherical shaped defect is analyzed depending on the deposition process used. The influence of a 3D defect position regarding the position of a line absorber is also shown.

Modeling of the influence of the defect position on the reflected intensity in EUV mask

In Extreme Ultraviolet Lithography, the electromagnetic modeling of the mask allows to determine the influence of the mask structure on the electromagnetic field. That makes it possible to take into account the presence of a defect modifying the multi-layer stack [1][2]. This paper presents the results of simulations, performed using a modal method, on the aerial image of the reflected intensity above the resist depending on the position of a defect with respect to an absorber pattern. These simulations allow to consider the influence of a defect not only on top of the structure but also everywhere inside the multilayer. The current method is the MMFE: Modal Method by Fourier Expansion. Modal methods are well adapted for EUV simulation mask due to materials and structure size.

In-line etching process control using dynamic scatterometry

In-line process control in microelectronics manufacturing requires real-time and non-invasive monitoring techniques. Among the different metrology techniques, scatterometry, based on the analysis of ellipsometric signatures of the light scattered by a patterned structures, is well adapted. Traditionally, the problem of defining the shape and computing the signature is dealt with modal methods and is called direct problem. On the opposite, the inverse problem allows to find the grating shape thanks to an experimental signature acquisition, and can not be solved as easily. Different classes of algorithms have been introduced (evolutionary, simplex, etc.) to address this problem, but the method of library searching seems to be the most attractive technique for industry. This technique has many advantages that will be presented in this article. However the main limitation in real-time context comes from the short data acquisition time for different wavelengths. Indeed, the lack of data leads to the method failure and several database patterns can match the experimental data. In this article, a technique for real time reconstruction of grating shape variation using dynamic scatterometry is presented. The different tools to realize this reconstruction, such as Modal Method by Fourier Expansion, regularization technique and specific software and hardware architectures are then introduced. Results issued from dynamic experiments will finally illustrate this paper.

Best focus shift mitigation for extending the depth of focus

The low-k1 domain of immersion lithography tends to result in much smaller depths of focus (DoF) compared to prior technology nodes. For 28 nm technology and beyond it is a challenge since (metal) layers have to deal with a wide range of structures. Beside the high variety of features, the reticle induced (mask 3D) effects became non-negligible. These mask 3D effects lead to best focus shift. In order to enhance the overlapping DoF, so called usable DoF (uDoF), alignment of each individual features best focus is required. So means the mitigation of the best focus shift. This study investigates the impact of mask 3D effects and the ability to correct the wavefront in order to extend the uDoF. The generation of the wavefront correction map is possible by using computational lithographic such Tachyon simulations software (from Brion). And inside the scanner the wavefront optimization is feasible by applying a projection lens modulator, FlexWaveTM (by ASML). This study explores both the computational lithography and scanner wavefront correction capabilities. In the first part of this work, simulations are conducted based on the determination and mitigation of best focus shift (coming from mask 3D effects) so as to improve the uDoF. In order to validate the feasibility of best focus shift decrease by wavefront tuning and mitigation results, the wavefront optimization provided correction maps are introduced into a rigorous simulator. Finally these results on best focus shift and uDoF are compared to wafers exposed using FlexWave then measured by scanning electron microscopy (SEM).

Optimization of double patterning split by analyzing diffractive orders in the pupil plane

In double patterning technology (DPT), two adjacent features must be assigned opposite colors, corresponding to different exposures if their pitch is less than a predefined minimum coloring pitch. However, certain design orientations for which pattern features separated by more than the minimum coloring pitch cannot be imaged with either of the two exposures. In such cases, there are no aerial images formed because in these directions there are no constructive interferences between diffractive orders in the pupil plane. The 22nm and 16nm nodes require the use of pixelized sources that will be generated using SMO (source mask co-optimization). Such pixelized sources while helpful in improving the contrast for selected configurations can lead to degraded contrast for configurations which have not been set during the SMO process. Therefore, we analyze the diffractive orders interactions in the pupil plane in order to detect limited orientations in the design and thus propose a decomposition to overcome the problem.

Resolving contact conflict for double patterning split

Double patterning (DP) is one of the main options to print devices with half pitch less than 45nm. The basis of DP is to decompose a design into two masks. In this work we focus on the decomposition of the contact pattern layer. Contacts with pitch less than a split pitch are assigned to opposite masks corresponding to different exposures. However, there exist contact pattern configurations for which features can not be assigned to opposite masks. Such contacts are flagged as color conflicts. With the help of design of manufacturing (DFM), the contact conflicts can be reduced through redesign. However, even the state of the art DFM redesign solution will be limited by area constraints and will introduce delays to the design flow. In this paper, we propose an optical method for contact conflicts treatment. We study the impact of the split on imaging by comparing inverse lithography technology (ILT), optical proximity correction (OPC) and source mask co-optimization (SMO) techniques. The ability of these methods to solve some split contacts conflicts in double patterning are presented.