Phillip L. Reu

Electron projection lithography mask format layer stress measurement and simulation of pattern transfer distortion

Electron projection lithography (EPL) is one of the leading candidates for the sub-65 nm lithography node. The development of a low-distortion mask is critical to the success of EPL. This article proposes and analyzes two new EPL mask formats described as a “corrugated-continuous membrane mask” and a “carbon-continuous membrane mask.” Novel process flows for the manufacture of these masks have been developed at Team Nanotec GmbH. Resonant frequency stress measurements of the ultrathin membrane bilayers were completed and subsequently used in the finite element simulation of the mask fabrication and pattern transfer. The new mask types have the benefits of the lower distortions of a typical continuous membrane mask, but maintain the advantage of the higher throughput stencil format because of the ultrathin films. In addition, the proposed masks remove the need for pattern splitting typically used with complementary systems.

Simulating the effects of pattern density gradients on electron-beam projection lithography pattern transfer distortions

The development of a low-distortion mask is critical to the success of the sub-0.1 μm lithography technologies. Electron-beam projection lithography (EPL) is one of the potential candidates for next-generation lithography. In order to minimize mask image placement (IP) errors, it is important to understand the factors that induce pattern distortions during mask fabrication and pattern transfer. The fabrication process flows for two EPL mask formats were numerically simulated and experimentally assessed for IP. This study included continuous membranes and stencil membranes for 1 mm ×1 mm and 1 mm ×12 mm window sizes on a 4 in. wafer. Both intramembrane (i.e., within a single window) and intermembrane (i.e., cross-mask) results are reported with excellent correlation between the finite element (FE) data and the experimental measurements. In this article details of the FE simulations are presented; an article by (M. Lercel et al., J. Vac. Sci. Technol. B, these proceedings) describes the corresponding experi...

Patterning-induced image placement distortions on electron beam projection lithography membrane masks

Membrane masks are needed for charged particle lithography and can include both stencil masks and masks with thin continuous membranes. Producing accurate image placement on membrane masks requires careful control of mask shape, pattern writing, and stress control of the mask materials. Pattern density and pattern density gradients also affect image placement (IP) control. This article discusses IP distortions on electron projection lithography masks caused by patterning the imaging layers with low and high density patterns and patterns with large gradients in the density. The process-induced distortion has been found to be largest with the largest vector distortion at the boundary when high pattern density gradients are present. The anisotropic stiffness of the unit cell also affects the process-induced distortion. Qualitatively, the results between continuous membrane and stencil masks show similar characters. The results provide distortion information that could be used to determine the maximum allowab...

Mask distortion issues for next-generation lithography

Identifying mask distortion issues of next-generation lithography (NGL) technologies are critical to the timely development of a successor to optical lithography at the sub-65 nm regime. This paper presents the results of finite element (FE) simulations that characterize the distortions induced during the fabrication of an electron-beam projection lithography mask. Employing a 100-mm prototype format, both global and local image placement errors were predicted with the numerical simulations. Equivalent modeling and submodeling techniques were used to provide detailed information on the distortion of individual circuit features of an SRAM device. The FE results were subsequently verified with experiments performed at the IBM/Photronics Mask Center of Competency. The data presented illustrate the benefits of modeling and simulation for design and optimization before implementation of NGL technologies.

Experimental and Numerical Studies of the Response of Photomask Hard Pellicles to Acoustic Excitation

To meet the stringent image placement error budgets for the insertion of 157-nm lithography in the sub-65 nm regime, photomask-related distortions must be minimized, corrected, or possibly eliminated. Sources of distortions include the pellicle system, which has been previously identified as a potential cause of image placement error. To characterize the many aspects of static pellicle-induced distortions, experiments have been conducted, and comprehensive finite element simulations have been performed for hard pellicle systems. The results of these benchmarking studies showed the capabilities of modeling and simulation to accurately predict static pellicle-induced distortions. In addition, the dynamic response of hard pellicles during exposure scanning was determined, taking into account both inertia effects and fluid / structure interaction. This paper focuses on the vibratory response of modified fused silica (hard) pellicles due to acoustic / dynamic pressure loadings during exposure scanning, studied both experimentally and numerically. A modal analysis was performed, the structural damping of the pellicle system was assessed, and a harmonic study was conducted to characterize the effects of acoustic excitation. The results obtained facilitate the timely establishment of viable hard pellicle designs and related standards for 157-nm lithography.

Experimental model verification of the thermal response of optical reticles

To extend optical lithography technology to the sub-100 nm linewidth regime, all mask-related distortions must be eliminated or minimized. Thermal distortion during the exposure process can be a significant contribution to the total pattern placement error budget for advanced photomasks. Consequently, several finite element (FE) models were developed to predict the thermal and the mechanical response of the optical reticle during exposure. This paper presents the experimental verification of the FE thermal models. In particular, the results of the numerical simulation were compared with the experimental data and excellent agreement was found.

Predicting critical dimension uniformity in advanced electron-beam projection lithography masks

Meeting the stringent budgets for both overlay errors and critical dimensions (CDs) for the insertion of next-generation lithographies requires that the sources of mask-related distortions be identified and minimized. Such sources include the mask fabrication process, electron-beam patterning, and exposure conditions. This article describes numerical simulations to characterize CD nonuniformities for the stencil format of the electron-beam projection lithography PREVAIL mask. Finite element (FE) submodeling and equivalent modeling techniques were used to investigate CD uniformity for a worst-case scenario. Whereas a global model is sufficient to predict mask overlay errors, submodeling and statistical analyses are necessary to characterize mask CD errors. The FE simulations illustrate that CD nonuniformities (from mask fabrication) can be limited to 1.0 nm, if the maximum membrane prestress is monitored and controlled. Such results comply with the CD error budgets set for the sub-65-nm lithography nodes.

Reduction of image placement errors in EPL masks

Minimizing mask-level distortions is critical to ensuring the success of electron projection lithography (EPL) in the sub-65-nm regime. Previous research has demonstrated the importance of controlling the stress in the patterned stencil membranes to minimize image placement distortions. Low-stress, 100-mm diameter EPL mask blanks have been patterned with a layout that simulates the effects of the cross-mask and intra-subfield pattern density gradients found in a realistic circuit design. Extensive IP measurements were made to illustrate how local subfield correction schemes can be used to reduce all mask-level distortions (regardless of pattern type) to less than 15 nm (3s). Combining membrane stress control with the use of repeatable and identical reticle chucking is expected to reduce EPL mask-level distortions to the values that will be needed for the 65-nm design node.

EUV Mask Stress Mapping by an Experimental and Hybrid Finite Element Technique

Image fidelity is one of the fundamental requirements in lithography and it is becoming more important as feature sizes shrink below 90 nm. Image distortion depends on the mask deformation caused by the intrinsic stress in the film-substrate system. To develop an understanding of stress generation and to control film quality, measuring film stress is essential. In recent years, research laboratories and industry have increasingly adopted indirect methods for determining film stress. All of these methods are based on the measurement of substrate deformation, and the film stress is calculated from the substrate curvature by the local application of Stoney’s equation. When the two principal stresses at each point in the film plane are not equal to each other and their distribution is not uniform, the local application of Stoney’s equation is invalid. Even though the accuracy of the measurement may be high, the stress determined may not be. An alternative technique based on numerical analysis has been developed. The limitations of using Stoney’s equation and the new stress measurement technique are discussed in this paper.

Film Stress Characterization Using Substrate Shape Data and Numerical Techniques

Intrinsic stress in a film-substrate system can have deleterious effects. To facilitate an understanding of stress generation and control film quality, measuring film stress is essential. In recent years research laboratories and industry have increasingly adopted indirect methods, which are usually based on the measurement of substrate deformation. The film stress is calculated by equations relating the stress to the deformation, such as the well-known Stoneys equation. However, when the two principal stresses at each point in the film plane are not equal and their distribution is nonuniform, the local application of Stoneys equation does not provide correct stress results. A numerical technique is presented, which overcomes these limitations and makes accurate stress determination possible.