Electron-beam Cell Projection Lithography Research Articles

Nanometric lateral scale development using an atomic force microscope with directly traceable laser interferometers

One-dimensional grating standards with sub-hundred nanometre pitches are required for calibration of nanometrological instruments. Nanometric lateral scales (design pitches: 100, 60 and 50 nm) for the calibration of nanometrological instruments were designed and fabricated by electron beam cell projection lithography. An offset-locked laser system consisting of an I2-stabilized He–Ne laser and a slave laser was installed in an atomic force microscope with differential laser interferometers (DLI-AFM) for the realization of a continuously, directly length-standard-traceable system and the pitches of the lateral scales were calibrated using the new DLI-AFM. The average pitches were quite close to the design pitches and the expanded uncertainties (k = 2) were less than 0.6% of the design pitches. The developed nanometric lateral scales are of sufficiently high quality and are candidates for certified reference materials (CRMs).

Highly Accurate Grating Reference Fabricated by EB Cell Projection Lithography

100-nm pitch size silicon grating pattern for calibration reference of electron-beam (EB) metrology systems was achieved. Grating patterns with pitch sizes of 240, 200, 160, and 100-nm in the x and y directions were delineated by EB cell projection writing system using grating stencil mask and these resist patterns were transferred in silicon substrate by dry etching. The pitch size error in the x and y directions was within 1 nm and the uniformity in 1.8×1.8-mm chip was under 3 nm within 3σ. More accurate or convenient calibration of the critical-dimension scanning electron microscope are expected compared with conventional calibration using 240-nm pitch reference grating fabricated by laser-interferometer lithography and anisotropic chemical etching.

Effects of accelerating voltage and pattern size on electron scattering by electron-beam mask

Electron scattering by electron-beam (EB) masks has been studied by a Monte Carlo simulation to investigate the influence of increasing accelerating voltage, of decreasing pattern size, and of using a thinner mask for a practical engineering application. The simulation study has revealed that there are two different exiting angle distributions; whether the most frequent exiting angle depends on L/S size or not. These are named interfeature scattering and intrafeature scattering, respectively. In order to analyze the electron scattering phenomena, we investigated the electron transmittance and absorbance by normalizing the mask thickness, using a Grün range. As a result, the EB mask was found to be categorazible into four function categories defined by the normalized mask thickness. They are pure absorber, scattering absorber, absorbing scatterer, and pure scatterer. This classification gives us important information for engineering design of the EB mask and for column design. For the EB mask, whose normalized mask thickness is more than 0.2, the transmitted electrons are scattered enough not to contribute to imaging in the angular and the energy distributions and high contrast can be obtained with some optimization of the angular limiting aperture. When the normalized mask thickness is less than 0.2, the limiting angle of the aperture may need to be decreased due to high electron transmittance, but optimization for different pattern sizes is not necessary in the case of the intrafeature scattering. The combination of thinner mask and an appropriate limiting aperture seems to be feasible and promising for future EB cell projection lithography to achieve higher resolution.

Modified mask methods for pattern accuracy enhancement in electron beam lithography

In projection-type lithography, delineated resist patterns differ from original mask shapes. In this article, aperture modification methods are investigated to enhance the pattern accuracy of electron beam cell projection lithography. The modified aperture is easily fabricated by decreasing the mask thickness. Although electrons penetrate the mask, enough high scattering contrast can be obtained. In dot delineation, star-shape apertures make better square dots near the resolution limit. In line-and-space delineation, dividing the pattern into small patterns of lower resolution reduces the electron current. This decreases Coulomb effects. In addition, Monte Carlo simulation results also confirm these results. As a result, these methods are expected to be important in the application of projection-type charged particle lithography.

Resolution analysis in electron-beam cell projection lithography system

We have experimentally analyzed the resolution in electron-beam cell projection lithography systems. The results indicated that the Coulomb interaction effect and the proximity effect are critical issues for resolution because of the larger beam current compared with the conventional variable-shaped beam. We achieved a practical resolution of 0.18 μm, which is enough for 1 Gbit dynamic random access memory (DRAM) fabrication, by adjusting the beam current. We have also found that there is some interaction between the individual line beams (streams). Also, the beam profile degradation in cell projection lithography systems depends not only on the beam current but also on the distance between the streams. Therefore, in order to obtain higher resolution, these effects must be considered more carefully than in the case of conventional variable-shaped beam lithography systems. A resolution of less than 0.15 μm, which is required for Gbit level DRAM fabrication, can then be available in cell projection lithography systems.

Electron-beam cell projection lithography: Its accuracy and its throughput

The cell projection electron-beam lithography system HL-800D has been developed for 0.2 μm ultralarge scale integration circuit and application specific integrated circuit use. This system is formed by a conventional variable shaped beam and cell beams for using five apertures. To achieve high resolution and high-stitching accuracy between the cell beams, and between variable shaped beam and cell beam, we developed the automatic tuning method for cell beams. Included in the method are: (1) rotation and magnification correction, (2) beam alignment for cell beams, (3) space-charge effect correction, and (4) mask position alignment. This method realized a system for easy handling of cell projection tuning. As a result, stitching accuracy under 0.05 μm and resolution under 0.2 μm for cell beams have been achieved. The contact hole layer and the wiring layer of a 0.35 μm DRAM pattern have been overlaid with an accuracy of 0.04 μm (maximum value).

An objective lens system for e-beam cell projection lithography

Abstract An objective lens system has been developed for electron beam cell projection lithography for quarter micron LSI. This system features are a 5 μm square maximum shot size, small deflection aberrations in a 5 mm field and high speed three stage deflectors. The experimental aberration coefficients agree well with simulated ones. As a result, cell patterns and 0.15 μm line and space patterns are delineated within a 5 mm field. This lens is applicable to memory and logic LSI fabrication.

Thermal Characteristics of Si Mask for EB Cell Projection Lithography

We evaluated the thermal characteristics of a single crystal Si mask for electron beam (EB) cell projection lithography by computer simulation. Due to the low thermal conductivity, the thermal characteristics of the Si mask are more critical than that of a conventional heavy metal mask. If the Si mask is assembled by the conventional method which thermal dispersion is due to only radiation, the temperature at the beam position rises drastically with the irradiation power of the EB. Moreover, the displacement caused by thermal expansion can not be ignored for a quarter-micron lithography. The Si mask assembly using thermally conductive adhesion and a new Si mask structure with ribs gives good thermal characteristics satisfying the necessary conditions for a high-throughput and highly accurate EB lithography.

Electron-beam cell-projection lithography system

An electron-beam exposure system HL-800D has been developed for the mass production of both quarter micron large-scale integrated memories and application specific integrated circuits (ASICs). To achieve a productive level of throughput, the system utilizes a cell-projection method combined with variable shaped method and a continuously moving stage at variable speed depending on the pattern density. The system is operated at a 50 kV acceleration voltage and a 1 μC/cm2 dosage. Three stage deflectors have been developed to assure high-speed deflection and highly accurate positioning. A fast pattern controller generates patern data at 200 ns shot-cycle-time with the positioning error correction and proximity effect correction. A high-speed ceramic XY stage and an automatic wafer loder have been developed. The system is operated by a workstation which also provides data conversion. The estimated throughput of the system is 11 wafers/h for 0.3 μm ASICs and 20 wafers/h for quarter micron memories.

Progress in EB-cell projection lithography

The throughput and accuracy of electron beam cell projection lithography are discussed. The method drastically reduces the number of e-beam exposure shots utilizing a specially shaped beam. A system designed to produce 64 and 256M-DRAMs is introduced. A 0.2 μm exposed pattern is also shown.

Electron-beam cell projection lithography: A new high-throughput electron-beam direct-writing technology using a specially tailored Si aperture

Electron-beam cell projection lithography realizes a high throughput capability suitable for ultra-large-scale integration (ULSI) manufacturing. This method makes it possible to drastically reduce the number of electron-beam exposure shots by utilizing a specially shaped beam. This shaped beam is created by an Si aperture which forms various shapes, coinciding with the array of each unit cell of an ULSI pattern. The aperture also forms a rectangular shape for conventional variable-shaped method in order to create random patterns. An aperture made of a single crystal of Si is fabricated using ULSI process techniques. As the reduction ratio of electron optics is larger than that of an optical stepper, pattern size errors caused by aperture inaccuracies can be reduced significantly.