Electron-beam Projection Lithography Research Articles

Wafer-scale nanofabrication of sub-5 nm gaps in plasmonic metasurfaces

Abstract In the rapidly evolving field of plasmonic metasurfaces, achieving homogeneous, reliable, and reproducible fabrication of sub-5 nm dielectric nanogaps is a significant challenge. This article presents an advanced fabrication technology that addresses this issue, capable of realizing uniform and reliable vertical nanogap metasurfaces on a whole wafer of 100 mm diameter. By leveraging fast patterning techniques, such as variable-shaped and character projection electron beam lithography (EBL), along with atomic layer deposition (ALD) for defining a few nanometer gaps with sub-nanometer precision, we have developed a flexible nanofabrication technology to achieve gaps as narrow as 2 nm in plasmonic nanoantennas. The quality of our structures is experimentally demonstrated by the observation of resonant localized and collective modes corresponding to the lattice, with Q-factors reaching up to 165. Our technological process opens up new and exciting opportunities to fabricate macroscopic devices harnessing the strong enhancement of light–matter interaction at the single nanometer scale.

Electron-beam lithography with character projection technique for high-throughput exposure with line-edge quality control

The high throughput of character projection (CP) electron-beam (EB) lithography makes it a promising technique for low-to-medium volume device fabrication with regularly arranged layouts, such as for standard-cell logics and memory arrays. However, non-VLSI applications such as MEMS and MOEMS may not be able to fully utilize the benefits of the CP method due to the wide variety of layout figures including curved and oblique edges. In addition, the stepwise shapes that appear because of the EB exposure process often result in intolerable edge roughness, which degrades device performances. In this study, we propose a general EB lithography methodology for such applications utilizing a combination of the CP and variable-shaped beam methods. In the process of layout data conversion with CP character instantiation, several control parameters were optimized to minimize the shot count, improve the edge quality, and enhance the overall device performance. We have demonstrated EB shot reduction and edge-quality improvement with our methodology by using a leading-edge EB exposure tool, ADVANTEST F7000S-VD02, and a high-resolution hydrogen silsesquioxane resist. Atomic force microscope observations were used to analyze the resist edge profiles’ quality to determine the influence of the control parameters used in the data conversion process.

Projection electron beam lithography for nanotechnology

The parameters of projection electron beam lithography were investigated and a resolution analysis was performed. It was shown that at least three optimization parameters must be considered in the process: the field size, the focal distance, and the angle on the target. Optimization was performed with allowance for the contribution of stochastic aberration.

Robust Control Design for Vibration Isolation of an Electron Beam Projection Lithography System

This paper describes vibration control for an electron beam projection lithography (EPL) system. Two kinds of disturbances should be considered for an EPL: load disturbances from the machine and ground disturbances from the environment. However, the suspension settings for insulating these two disturbances conflict with each other. Therefore, we propose a double-layer optical table and apply disturbance response decomposing (DRD) techniques to independently control the disturbances. We use a passive control structure to isolate the ground disturbances, and an active control structure to suppress load disturbances. In addition, symmetric transformation is applied to decouple a full optical table into bounce/pitch and roll/warp half-table models, which can be further decoupled into quarter-table models to simplify controller design. Finally, we apply robust control techniques to design active controllers. From both simulation and experimental results, the designed H∞ robust controllers are proven effective in reducing EPL system vibrations.

Development of Stage Temperature Control for Vacuum Lithography Tool

It has become a recent trend to apply a special environment into a lithography tool, for instance, immersion, EPL (Electron beam Projection Lithography) and EUVL (Extreme Ultra Violet Lithography) in order to exposure less than 22nm line and space at semiconductor industry field. For a cooling system that a heater is surrounded by metal can and a coolant is supplied into the can by using movable tubes, not only flexibility, damping, durability, particle protection and anti-liquid, etc, but also pressure-resistance, low outgasing and low leak from connections, etc might be strongly taken into account for vacuum lithography. Especially, because a coolant leak is very critical for EUVL in which a partial pressure of several molecular is strictly restricted, a special tube and joint have to be required in the case of liquid cooling system. On the other hand, a gas cooling can be one candidate because of a lower leak damage, however it is necessary to compensate its low cooling efficiency of gas system by using temperature feedback control. In this paper, two guide lines of a cooling mechanism for wafer and mask stage of vacuum compatible tool, a liquid cooling and gas cooling system are introduced.

Method of improving the quality of nanopatterning in atomic image projection electron-beam lithography

The authors demonstrated successful nanopatterning using a high resolution lattice image obtained by transmission electron microscopy (TEM) as a mask signal [H. S. Lee et al., Adv. Mater. (Weinheim, Ger.) 19, 4198 (2007)]. In this process, the quality of the patterning result is critically dependent on the lattice image, which, in turn, is critically dependent on the quality of the mask. It is often noted that the quality of the mask prepared by the conventional TEM sample preparation technique is far from perfect, which critically limits the quality of patterning. In this work, they first discuss the various origins of the noise signal generated by the mask and then introduce a special type of objective aperture (noise reduction aperture) to remove the noise signal. The effect of the noise reduction aperture, designed for the Si [110] zone axis, is experimentally demonstrated.

Monte Carlo Simulation of Projection Electron Beam Lithography

We have calculated angular and energy loss distributions of electrons transmitted through masks for electron projection lithography by using a Monte Carlo simulation method. The angular and energy loss distributions are much wider in the mask stack than those in the membrane layer. The large non-scattering and non-energy-loss probabilities are also found for the membrane layer. High contrast image can thus be achieved within a small size of aperture.

Mask Charging Phenomena during Electron Beam Exposure in the EPL System

Charging phenomena of a mask material during electron beam exposure are analyzed in an electron beam projection lithography system. First, the three-dimensional charge deposition distribution by the electron beam irradiation is obtained. Next, in every time step, the distributions of the accumulated charge and the potential are obtained considering the current flow due to the diffusion and the drift. As a narrow bridge pattern defined in a 5 microm x 5 microm area is assumed to lay out all over the field of 1 mm x 1 mm and the potential is grounded at the circumference of the field (1 mm x 1 mm x 1 mm), the saturated potential distribution is obtained at the central 5 microm x 5 microm area in the field. The maximum potential attained is around 4.23 microV at the center of the bridge, if the accelerating voltage of the electron beam is 100 kV, the current density is 10 A/cm2, and the material of the mask is the intrinsic Si. The contribution of the charging may be negligible to the electron beam with such a high accelerating voltage, which is going through the opening beside the bridge pattern in the projection lithography.

Thermal analysis of diamondlike carbon membrane masks in projection electron-beam lithography

Diamondlike carbon (DLC) membrane masks for high-throughput projection electron-beam lithography have an important advantage over stencil masks: a single mask can be used to print donut-shaped patterns. At a membrane thickness on the order of a few tens of nanometers, thermal problems could occur. The temperature rise of a membrane mask was simulated with various exposure parameters. Membrane and scatterer thickness, resist sensitivity, and pattern coverage were varied. It was found that global temperature rise after exposure of a chip is on the order of 0.005 °C, while temperature rise in a membrane area under exposure reaches 2.54 °C. Simulation results were compared to those on a silicon stencil mask. It was found that the global heating is less on a DLC membrane mask.

Impacts of 30-nm-thick resist on improving resolution performance of low-energy electron beam lithography

Exploring the resolution performance of various electron beam lithography with the acceleration voltage of 2–100kV showed that line edge roughness (LER) and resolution limit of resist patterns was in linear relation with blur of latent image profile. Reducing resist thickness is effective in decreasing the blur of low-energy electron-beam proximity projection lithography (LEEPL) because 47% of the blur is due to scattering of 2keV electrons in 70-nm-thick resist film. The impact of thinning the resist thickness on the LER and the exposure latitude in LEEPL was investigated by simulation and experiment. A Monte Carlo simulation shows that the LER of a 100-nm-width space pattern decreases to 5.5 from 10.4nm with reducing the resist thickness to 20 from 70nm. Exposing the photosensitive polyorganosilazane resists (Clariant (Japan) K.K.) by LEEPL-3000 underpinned that latent image quality was improved by thinning the resist. The exposure latitude of 80-nm-diameter dense contact holes increased to 25 from 16% with reducing the resist thickness to 30 from 50nm. It is necessary for taking an advantage of the improved latent image of the 30-nm-thick resist in resolving the 90-nm-pitch patterns to mitigate the edge roughness probably caused by the properties of a highly interfacial resist.

Resolution-limiting factors in low-energy electron-beam proximity projection lithography: Mask, projection, and resist process

The resolution-limiting factors in low-energy electron-beam proximity projection lithography were analyzed quantitatively using the blur of a Gaussian-shaped latent image (σQBP) as the resolution index. σQBP is the square root of the sum of squares of the factors, such as electron scattering and resolution performance of resist. The resolution limit of 45 nm for isolated patterns and the resolution of 70 nm for practically used periodic patterns with 10% exposure latitude were achieved at σQBP of 49 nm. Eliminating a crossover in the electron optics decreased the factor depending on the gap between a mask and a wafer to 19 nm at a 40 μm gap. Because of the intensive studies on multilayer processes, the factor attributed to the resolution performance of thin resist dropped from 58 to 26 nm. Reduction in the blur due to electron scattering, 34 nm in the case of a 70-nm-thick resist film and 2 keV electrons, must be considered for the 45 nm technology node.

Challenges to Ultra-thin Resist Process for LEEPL

An ultra-thin resist process is indispensable for low-energy electron-beam proximity projection lithography (LEEPL) because it uses 2-kV-accelerated electrons with small penetration depth. 70- nm-thick chemically-amplified resists for a tri-layer process were developed with the consideration of the interaction of a polymer with a spin-on-glass material, showing the resolution of 140-nm- pitch contact holes. Application of the tri-layer process developed for LEEPL to making via holes in a 90-nm-node back-end-of-line process proved that the ultra-thin resist was lithographically useful in terms of resolution and etching tolerance. Exploring the resolution performance of electron beam lithography showed that line edge roughness and resolution limit of resist patterns was in linear relation with blur of latent image profile. Reducing the resist thickness is effective in enhancing the resolution of LEEPL because 47 % of the blur is attributed to electron scattering. A Monte Carlo simulation shows that the blur caused by the electron scattering decreases 41 %, to 20 nm from 34 nm, by reducing the resist thickness to 30 nm from 70 nm.

The stitching error dependence of the resist pattern accuracy for electron beam projection lithography

The stitching error dependence of the resist pattern accuracy for electron beam projection lithography is presented. The terrace pair is employed to divide the pattern in order to control the uniformity of critical dimension (CD) within large stitching margin. The model of electron scatting is supposed in the simulation. The results show that the CD and pattern displacement are within budget if the overlapping and transverse shifting of pattern are controlled within ±20 and ±30 nm, respectively, for 100 nm node generation.

Thermal analysis of projection electron beam lithography using complementary mask exposures

In projection electron beam lithography (EBL), most of the e-beam energy is deposited in the resist and substrate as heat. The temperature rise and its effect on critical dimension (CD) variation and placement error were studied at 100 kV projection EBL for exposure conditions used on a full scale production tool. A few writing strategies were considered. Analytic estimation of temperature rise was followed by numerical simulations using the temperature simulation software tool. Local temperature increases by a maximum of 10.7 °C was found at 5 μC/cm2 exposure dose, which corresponds to a few nanometers CD variation and can be over 10 nm depending on the resist and beam blur. Global substrate heating leads to a few tens of nanometers placement error that may suggest a need for better wafer cooling or active placement correction.

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.

Wafer heating analysis for electron-beam projection lithography

Electron projection lithography (EPL) is one of the principal next-generation lithography technologies for the sub-65 nm regime. To satisfy the stringent resolution requirements, all image placement errors must be characterized and minimized. These include the distortions of the device wafer during exposure. The wafer absorbs beam energy which produces temperature increases and thermomechanical strains that directly contribute to pattern-placement errors, stitching errors between adjacent subfields, and image blur. Thus, CD control and pattern overlay will be directly affected. In this article, the thermomechanical distortions caused by wafer heating in the EPL system have been simulated using finite element models that include the effects of the interaction between the wafer and chuck.

Estimation of Optimum Electron-Beam Projection Lithography Mask Biases Taking Coulomb Beam Blur into Consideration

In electron-beam projection lithography, a large field is exposed by one dose, and the mask biasing has been proven to be one of the most suitable methods for proximity effect correction. In this study, we obtained the exposure intensity distribution (EID) function through an exposure experiment with a 100 kV point beam exposure tool and expressed it as the sum of seven Gaussian functions. We calculated the exposure dose distribution in the line-and-space and isolated-line patterns as convolutions of the 7-Gaussian EID function and mask patterns, and the predicted line widths agreed well with the experimental results. The range of mask biases determined using the 7-Gaussian EID function for the gate and metal interconnect layers of 65-nm-node ASIC devices were ±22 nm and ±43 nm, respectively.

Stencil Mask Technology for Electron-Beam Projection Lithography

The development of silicon stencil masks for cell projection lithography (CPL), electron projection lithography (EPL), low-energy electron-beam-proximity-projection lithography (LEEPL), and low-energy electron-beam lithography (LEB) is described. To satisfy the required pattern accuracy of the stencil masks, such as critical dimension and image placement, we developed a high-aspect stencil pattern formation technique to achieve a side-wall angle of over 90 deg and a membrane stress control technique using a stress-correction layer. In addition, for fabricating low-magnification stencil masks, we developed a new mask substrate with a sputtered scattering silicon membrane and a sputtered intermediate stopper layer as an etching stopper. We selected CrNx as the intermediate layer material, which demonstrated high performance in stencil mask fabrication. By improving the CrNx film qualities, its durability to dry and wet etching of backside silicon was improved. In order to improve the pattern image placement accuracy caused by membrane stress changes during stencil pattern formation, we developed a new and simple technique using a stress correction layer, which also functions as a silicon deep-etching mask. By using this functional layer, the total stress change in the pattern field in a mask pattern formation process was reduced to within 5 N/m, which corresponds to less than a 10 nm pattern image placement (IP) error.

High-Speed Proximity Effect Correction System for Electron-Beam Projection Lithography by Cluster Processing

A fast proximity effect correction system for electron-beam projection lithography is developed. The following techniques are investigated to improve the performance; the patterns in each 250-µm-square subfield into which the original layout data is split, are corrected in parallel by a cluster system. Several subfields are processed at a time on one cluster node. Actual device data of a 70 nm system-on-a-chip is corrected by our proximity effect correction system to demonstrate its performance. The processing time for the proximity effect correction is reduced to 3.86 h from 60 h not only by using a cluster system with a 248 MHz processor ×8 nodes but also by processing 4×4 subfields simultaneously.

Progress in Resists Development for EPL (Electron Beam Projection Lithography)

Electron beam (EB) lithography, together with ArF, F2 and EUV (Extreme Ultra-Violet), is one of the most promising candidates for the next generation 65 nm node and below from the viewpoint of resolution and DOF (depth of focus). However, since EB lithography has a disadvantage in throughput compared with optical lithography, it is generally used for mask and ASIC (Application Specific Integrated Circuits) device manufacturing. The application for it in mass production has been a difficulty. Recently, EPL (Electron Beam Projection Lithography) and LEEPL (Low Energy Electron Proximity Lithography) are being developed to improve the throughput problem of EB. In this paper, we investigated the subject of high sensitivity for imaging resist, which is one of the current topics of EPL. We resulted in an imaging resist which has high sensitivity and small line edge roughness by applying a specific additive and specific resin. Furthermore, we also developed an imaging resist which has high sensitivity and excellent resolution by applying the acid generator which controlled the strength of acid, and diffusion.