Variable Shaped Electron Beam Research Articles

Study and comparison of resist characteristics for different negative tone electron beam resists

In this work, four types of negative electron beam resists are investigated. Electron beam lithography (EBL) experiments were conducted using EBL system ZBA23 (Raith) with the variable-shaped electron beam cross-section at 40 keV electron energy. Important electron beam resist characteristics such as sensitivity, dissolution rate, aspect ratio and sidewall developed profiles in the chemically amplified resist (CAR) SU-8 2000, non-CARs ma-N 2410 and ARN-7520, and inorganic negative resist HSQ XR-1514 are studied and compared. This study was motivated by the selection of a suitable resist for practical use such as large area gratings fabrication for optoelectronics.

Study on physical model of resist surface charge in multi-beam mask writer and single variable-shaped beam writers

NuFlare Technology, Inc. has developed the charge effect correction (CEC) system, in which the predicted placement errors caused by resist surface charge effect are compensated. Due to good reproducibility of these exposure-dose-dependent and time-dependent behaviors, CEC has successfully improved image placement accuracy in single variable-shaped electron beam mask (EBM) writers. The physical mechanism of resist charging, however, has remained unclear. Considering large difference in beam current densities between EBM and multi-beam mask writers (MBM), it is concerning that unexpected charge phenomena occur. Therefore, it is required to reveal the mechanism and to show the resist charging is still predictable in MBM by a common charging model. In order to provide validity of CEC consistently with EBM and MBM, the surface charge densities caused in EBM writers and MBM-2000 were experimentally evaluated from those placement errors. The charge densities strongly depend on the resist thickness meanwhile they do not change significantly by the writers. Furthermore, equations to reproduce those experimental results are proposed in terms of a physical model, which takes into account low-energy secondary electrons drawn to resist surface by an already-existing surface charge and vertical flow of the charge in the resist.

Point spread function and total process function measured for a variable-shaped electron beam system

In the field of electron beam lithography, accurate exposure requires a precise proximity effect correction (PEC). The most common approaches for PEC such as dose modification, shape modification, and hybrid models require the evaluation of the point spread function (PSF) and, in view of real applications, the measurement of the total process function (TPF) considering both tool behavior and process effects. In contrast to research tools equipped with a Gaussian beam column, variable-shaped beam (VSB) systems are established in the commercial field, e.g., for mask fabrication. In this paper, the authors present an approach to measure PSF and TPF on a VSB lithography tool. This method is based on the well known point exposure distribution measurement technique early introduced for Gaussian beam systems by Rishton and Kern [J. Vac. Sci. Technol., B 5, 135 (1987)]. The approach is enhanced by application of specific features provided on a VSB system. The interpretation of the experiment was enabled by a comparison of the measured data with calculations of the energy deposition in the resist using Monte Carlo simulations. Results are presented for hydrogen silsesquioxane and Novolac negative tone resists that are used on standard i-line photomasks.

Characterization for the photomask fabrication based on a high-resolution technique with a non-chemically amplified resist and a post-exposure bake

A high-resolution technique has been developed for the fabrication of photomasks for 10nm logic nodes and beyond. Current mask manufacturing techniques use a chemically amplified resist (CAR) material that has a complex mechanism of acid generation; this complicates the criteria for selecting the polymer and the quencher for industrial purposes. It is therefore important to validate non-CAR materials as alternative solutions for mask fabrication. In this research, we used diluted ZEP520A as a non-CAR material in conjunction with a JBX9000 variable-shaped electron-beam (VSB) lithography tool. Additionally, a post-exposure bake (PEB), routinely used in mask fabrication, was also applied. We investigated the effects of the PEB temperature on the fabrication of masks from non-CAR resists, and we demonstrated the feasibility of using a PEB as a high-resolution technique. The critical dimensions (CDs) for 1:1 line-and-space, isolated space, and isolated line patterns on a diluted ZEP520A resist were measured and showed a shrinkage, an extension effect, and retention of the integrity of the shape after the PEB process, respectively. Furthermore, the line-edge roughness (LER) of the 1:1 line-and-space and isolated space CDs was improved by approximately 40% by optimizing the PEB temperature. We investigated the process characteristics of this PEB annealing effect by examining the hardness of the cured resist with and without exposure to PEB at various temperatures, with the aim of elucidating the underlying mechanism. Optimizing the PEB temperature of the non-CAR increased the resist contrast, annealing the resist and improving the LER. This permitted us to demonstrate an advanced fabrication technique capable of high resolutions of the order of 20nm. The insights gained from the optimization of the PEB process might be useful in advanced methods of fabricating masks of the next generation.

Fabrication of a high-resolution mask by using variable-shaped electron beam lithography with a non-chemically amplified resist and a post-exposure bake

A high-resolution technique was developed for fabrication of photomasks for 10-nm-technology nodes and beyond. Current mask manufacturing techniques use a chemically amplified resist (CAR) photoresist material that has a complex mechanism of acid generation that obscures the criteria for selecting the polymer, quencher, and crosslinker for industrial purposes. It is therefore important to validate non-CAR materials as alternative solutions for mask fabrication. In this research, diluted ZEP520A was used as a non-CAR material in conjunction with a JBX9000 electron-beam lithography (EBL) tool. Additionally, a post-exposure bake (PEB) is normally used in mask fabrication. The PEB method was also used in this research, and we investigated its temperature dependence. Critical dimensions (CDs) of 1:1 line-and-space, isolated space, and isolated line patterns on a diluted ZEP520A resist were measured and showed CD shrinkage, an extension effect, and retention of the integrity of the shape after the PEB process. A resolution of the order of 20nm was attained and the insights gained from optimization of the PEB process might be usefully applied in advanced methods for fabricating masks of the next generation.

Evaluation of direct patternable inorganic spin-on hard mask materials using electron beam lithography

Two inorganic non-chemically amplified resists (XE15CB and XE15IB) were investigated. The resists are based on hafnium oxide sulfate and spin-cast from aqueous solution. They differ in sensitivity and resolution. In contrast to other studies, the materials were coated for the first time on 300 mm wafers and exposed on a 50 kV VISTEC SB3050DW variable shaped electron beam direct writer in a near-production environment at Fraunhofer CNT. In this paper, the evaluation of this resist family and its feasibility for production will be discussed. The resists were evaluated in terms of contrast, sensitivity and resolution. The process characteristics required for CMOS manufacturing such as delay stability were also examined. Furthermore, by exposing a large SRAM pattern (design CD of 22 nm), the exposure of a real application pattern was demonstrated.

Plasmonic properties of aluminum nanorings generated by double patterning

In this Letter we evaluate a technique for the efficient and flexible generation of aluminum nanorings based on double patterning and variable shaped electron beam lithography. The process is demonstrated by realizing nanorings with diameters down to 90 nm and feature sizes of 30 nm utilizing a writing speed of one ring per microsecond. Because of redepositions caused by involved etching processes, the material of the rings and, therefore, the impact on the plasmonic properties, are unknown. This issue, which is commonly encountered when metals are nanostructured, is solved by adapting a realistic simulation model that accounts for geometry details and effective material properties. Based on this model, the redepositions are quantified, the plasmonic properties are investigated, and a design tool for the very general class of nanofabrication techniques involving the etching of metals is provided.

Optimized electron beam writing strategy for fabricating computer-generated holograms based on an effective medium approach

Recent research revealed that using the effective medium approach to generate arbitrary multi-phase level computer-generated holograms is a promising alternative to the conventional multi-height level approach. Although this method reduces the fabrication effort using one-step binary lithography, the subwavelength patterning process remains a huge challenge, particularly for large-scale applications. To reduce the writing time on variable shaped electron beam writing systems, an optimized strategy based on an appropriate reshaping of the binary subwavelength structures is illustrated. This strategy was applied to fabricate a three-phase level CGH in the visible range, showing promising experimental results.

Determination of best focus and optimum dose for variable shaped e-beam systems by applying the isofocal dose method

Electron beam direct write (EBDW) provides high resolution for device and technology development. A new variable shaped beam system with improved electron optics was introduced, which features the capability for the 32nm node. Because of the limited resolution of commercially available chemically amplified resists at this node, it is important to determine a stable and optimum resist process window. To compare a process window under different premises, a universally applicable and low error-prone method is needed. The isofocal dose method is investigated with regard to these properties for its use in EBDW. Experiments were performed on 50kV variable shaped electron beam direct writers using the new electron-optical column SB3050 DW (Vistec Electron Beam GmbH). Exposures are performed at different sites in Dresden (Fraunhofer CNT/Qimonda Dresden), Jena (Vistec) and Stuttgart (IMS Chips); also patterns are exposed on different layer stacks at one site. The strong need for a process window can be fulfilled by the isofocal dose method, which will be shown by contour plots.

Measurements of Variable-Shaped Electron Beams with Solid-State Detector and Scattering Aperture

A highly accurate method for measuring beam properties in a variable-shaped electron beam (VSB) system has been developed. This method is based on a knife-edge method with a solid-state detector (SSD) and scattering apertures. In VSB system, it is necessary to measure both beam profile and beam position for a long time. To meet this requirement, many aperture marks on a silicon membrane were prepared in a measurement unit. Using this unit, the accuracy and stability of beam-size and beam position measurements were evaluated in VBS system (HL-7000D, Hitachi-HITEC). As a result, the repeatability error for beam size was obtained to be smaller than 2 nm (3σ) and the repeatability error for beam position was obtained to be 0.82 nm (3σ). Moreover, a multitude of repeat experiments showed that this measurement unit can be used for more than ten years. Consequently, it was confirmed that this measurement method is useful for the high accuracy of a VSB system.

Variable Shaped Electron Beam Lithography for Fabrication of Very Large-Scale Integrated Circuits : An Example of Research and Development in the Information Science Laboratory, The Institute of Physical and Chemical Research

Variable Shaped Electron Beam Lithography for Fabrication of Very Large-Scale Integrated Circuits : An Example of Research and Development in the Information Science Laboratory, The Institute of Physical and Chemical Research

Advantageous Exposure of X Architecture Patterns when Using a Variable-Shaped Beam Tool

The concept of introducing diagonal interconnects in chip manufacturing offers several advantages and is known as X architecture. We investigated the fabrication of such patterns for high-resolution direct writing applications using variable-shaped electron beam lithography. The variable-shaped beam offers throughput advantages for Manhattan patterns. These advantages apply also to X architecture patterns, provided slanted rectangular and triangular shots can be used for pattern generation. This specific shot generation method has already been incorporated in Leica SB350 electron beam writing tools, so that it has become possible to significantly reduce the number of shots required for exposing X architecture layouts. The resolution is not affected by the pattern orientation. 130 nm pitch lines and spaces with axis-parallel and slanted orientations were successfully exposed in a 250-nm-thick NEB-33 resist.

Comparative study of resolution limiting factors in electron beam lithography using the edge roughness evaluation method

Resolution capabilities of a Nikon 100 kV experimental column of electron beam (EB) projection lithography, a proof of concept column of low energy electron beam proximity projection lithography (LEEPL), and a variable-shaped EB direct writer were compared using a negative-type chemically amplified resist. Line edge roughness and resolution limit of an isolated line were 2.8 and 40 nm for the Nikon experimental column, and were 4.9 and 52 nm for the EB direct writer at the resist thickness of 250 nm, while they were 10.7 and 61 nm for LEEPL, and were 9.6 and 57 nm for the EB direct writer at the resist thickness of 70 nm. Quantitative influences of resolution limiting factors such as electron-optical beam blur and resist performance were analyzed using the edge roughness evaluation method. The analysis shows that the Nikon experimental column has an ability to resolve 35 nm patterns if resolution performance of the resist is 10% improved. In the high-throughput EB stepper system with a large beam current, it is necessary for resolving 50 nm patterns to keep the electron-optical beam blur under 36 nm. It is known about LEEPL that a necessary and sufficient condition for realizing 50 nm patterns is to develop a 50 nm thick resist having the same resolution performance as the conventional resist dedicated for the use with thicker film thickness, and to obtain a 50 nm pattern mask having the same aperture roughness as the conventional mask with 70 nm patterns.

Precise Delineation Characteristics of Advanced Electron Beam Mask Writer EB-X3 for Fabricating 1× X-Ray Masks

This paper discusses the delineation characteristics of a state-of-the-art variable-shaped electron-beam (e-beam) mask writer called the EB-X3, which was developed both to write 100-nm-node patterns on X-ray membrane masks and also to study the feasibility of the mask technology. For a 1-Gb dynamic random access memory (DRAM) pattern (22 mm×22 mm chip), the best image placement (IP) accuracy obtained so far is 3σ(X, Y) = (5.6 nm, 8.4 nm) and maximum Δ(X, Y) = (4.2 nm, 7.3 nm). IP values less than 15 nm are reproducible. Precise temperature control and a three-point support pallet (4-inch membrane warpage: <100 nm) are the keys to this excellent IP. A resolution of 50 nm was obtained for line-and-space patterns. Good critical dimension control of less than 10 nm and small proximity effects were obtained with ZEP resist and normal-hexyl acetate developer. A total overlay accuracy for proximity X-ray lithography of better than 40 nm was achieved at the 1-Gb-DRAM level using X-ray masks with a 19-nm overlay accuracy (gate to contact) fabricated using the EB-X3.

A new figure fracturing algorithm for variable-shaped EB exposure-data generation

This paper discusses a new figure fracturing algorithm to be used in input data generation for the variable-shaped EB (electron beam) exposure system. In the input data generation of this kind of system, processing is required in which the individual polygons composing the mask pattern are represented as a set of trapezoids (figure fracturing). The figure fracturing considered in this paper differs from those in the conventional methods, in that the generation of a narrow trapezoid as well as the partitioning of the critical part is suppressed, in order to improve the accuracy of LSI mask fabrication. In this paper, the problem is solved as the fracturing of the composite rectangle into a set of rectangles by removing slant edges from the polygon. The relation graph is introduced to represent the relation among the partitioning line candidates. A graph-based algorithm is presented that efficiently selects the adequate partition lines from all possible candidates. The proposed method is applied to the mask fabrication for 64 M and 256 M DRAM, and a satisfactory fractured result is obtained in a practical execution time. © 2000 Scripta Technica, Electron Comm Jpn Pt 3, 83(8): 87–102, 2000

Cellector: A new software tool for the support of cell projection in combination with variable shape electron beam lithography for mask and direct write applications

A new software tool for the support of cell projection in combination with variable shape electron beam lithography for mask making and direct write application is described. The program has been designed and developed for the WePrint 200, a new variable shape electron beam lithography system from Leica Microsystems Lithography GmbH, which includes a powerful cell projection unit. The WePrint 200 allows the use of 20 different mini-reticles with one set of apertures. Starting from the chip layout, Cellector calculates the layout structures which are potential candidates for cell projection and exposable using mini-reticles. The determination of the mini-reticle candidates takes place with consideration of the pattern neighbourhood relationship and the hierarchically layout description. For each mini-reticle, the program also calculates statistical values, like the number of calls per mini-reticle and the amount of flash reduction. These values help the user select the optimal mini-reticle set for the cell projection step. With the application of PROXECCO^1 The benefit of throughput enhancement using cell projection is directly correlated with the regularity of the chip layout and with the reduction of flashes by using mini-reticle. In the paper throughput enhancements by application of cell projection are given for different kind of chips.

Distributed, multiple variable shaped electron beam column for high throughput maskless lithography

The ultimate resolution obtainable with focused electron beams is, for practical purposes in lithography, unlimited. Existing e-beam lithography systems are too slow to be practical for high volume manufacturing of semiconductor devices, however. The usable current in probe forming systems is limited by the stochastic Coulomb interaction in the beam path, which causes loss of resolution at high current. This is due to the need to pass all of the writing current through an aperture. Distributed systems, by contrast, do not suffer from this problem, as the current is spread over a large volume. The purpose of this article is to propose a distributed system, employing multiple, variable shaped beams for direct write (maskless) lithography. We call this system DiVa, to emphasize the key attributes of distributed writing current, and variable beam shaping. It utilizes a planar cathode, patterned with a rectilinear array of square emitters. Focusing is accomplished by a uniform, axial magnetic field, oriented along the optic axis. This transfers a one-to-one image of the emitters onto the writing surface. Deflection plates between adjacent rows of beamlets effect motion in one Cartesian axis, while mechanical translation of the stage effects motion in the orthogonal axis. Theoretical resolution is diffraction limited to 4.6 nm at 50 kV. Experimental results are presented which demonstrate the first order optics using a rudimentary apparatus.

Electron Beam Direct Writing Techniques for the Development of Sub-Quarter-Micron Devices

Novel techniques for electron beam (EB) direct writing are established for the development of sub-quarter micron devices. These techniques include 1) use of highly stable processes for positive and negative chemical amplification resists, 2) precise pattern size control by way of proximity effect correction using the pattern-area-density method, 3) a practical improvement technique for pattern size error at the exposure-field boundary, and 4) a novel alignment technique for a substrate covered with a heavy metal film. Successful fabrication of sub-quarter-micron test devices indicates that variable-shaped EB direct writing using these techniques is the best tool for research and development of advanced devices.

A 100 kV Electron Gun for the X-Ray Mask Writer, EB-X2

A 100-kV thermal-emission electron gun was developed for the X-ray mask writer EB-X2, which employs a variable-shaped electron beam with a beam edge resolution of 20 nm. The optimized design of the EB-X2 electron optical system requires that the electron gun has crossover diameters of 50 µ m and an optical length of less than 100 mm. So the crossover diameters of the gun were accurately calculated with an electron ray tracing program, and a gun with the required crossover diameters and optical length was designed. The gun was constructed, and the crossover diameters were measured. The measured values agree well with the calculated ones, and this confirms that the gun is suitable for use in the EB-X2 electron optical system.

Triangular-variable-shaped beams using the cell projection method

A triangular variable shaped electron beam lithography method using the cell projection method is proposed. The shaped beams are formed by a conventional rectangular beam method and variable triangular apertures. This method has three significant characteristics: The plural kind of triangles can be made. This increases the pattern flexibility and decreases the number of shots. The position of one corner of the triangles is automatically determined. Therefore, there is no fluctuation of the triangular beam position and the size of the triangle can be varied with high precision. The triangular apertures are located such as to minimize the deflection distance. This realizes high speed and high precision beam formation. With these characteristics, this method is expected to be used for high speed and high precision mask fabrication.