Electron-beam Cell Projection 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.

Variable cell projection as an advance in electron-beam cell projection system

As an advance in the electron-beam cell projection system, we have developed a deflection control and correction unit of the variable cell projection (VCP) system, which can increase the variety of projected images through a block of patterns on a CP mask. We estimate the extent of the minimum spacing between two patterns distinguished by partial illumination to be 58 nm on a wafer, which is due to the beam blur of 28 nm and the illuminating beam edge rotation of 6 mrad relative to the CP mask patterns. Exposure results show that the VCP can actually distinguish the patterns separated by 80 nm on a wafer. We also show that the VCP can improve the uniformity of linewidths of exposed patterns by about 5 nm compared with the simple VSB method.

Fine Pattern Fabrication below 100 nm with 70 kV Cell Projection Electron Beam Lithography

Cell projection electron beam lithography has been developed to achieve high throughput, and this system is now available commercially. However, the cell projection method uses a beam current that is much higher than that of the variable shaped beam method, and the Coulomb interaction caused by electrical repulsion (the coulomb effect) is thus much stronger. It is important to control the Coulomb effect in fine pattern fabrication because it causes beam blur that degrades resolution. This paper describes the effects of the acceleration voltage on the resolution of fine patterns below 100 nm as determined through experiments and simulations. Using an electron beam (EB) lithography system with 70 kV acceleration, we made 120 nm pitch lines and spaces (L&S) patterns by the cell projection method. Throughput was improved by using an acceleration voltage higher than the usual 50 kV acceleration. Because the Coulomb effect under 70 kV acceleration is weaker, we could use a higher beam current density or a larger cell projection size. The 70 kV acceleration is particularly useful for fine patterns and enables throughput twice that possible with 50 kV acceleration for 70 nm L&S pattern delineation.

Characteristic Variation of Exposure Pattern in Cell-Projection Electron-Beam Lithography

The characteristic variation of an exposure pattern with various optical parameters is calculated in a typical cell-projection lithography system by using an electron trajectory simulation. The Coulomb interaction effect among electrons in the beam is calculated which determines the pattern blur at the wafer surface. The acceleration voltage is 50 kV, the exposure pattern is a series of line and space, and the field size is 5 µm×5 µm. As the beam current density varies from 2 to 13 A/cm2 at the specimen surface, the pattern blur is evaluated in parameters of (1) the probability of electrons to be in the designed pattern, (2) the full width at half maximum and (3) the contrast of the electron density distribution at the wafer surface, as the line width varies from 0.13 to 0.2 µm.

部分一括露光方式電子ビームリソグラフィ用シミュレータの開発

We developed a Monte Carlo simulation of electron trajectories in a cell-projection electron beam lithography optical system. In the simulation the emission angle, the velocity and the emission timing of electrons from the electron gun are taking into account. The Coulomb interaction among electrons in the beam is calculated. There are five lenses in the system. Assuming that the stencil mask pattern is a series of line-and-space, the exposure pattern is obtained when the current densities are 5, 10, and 13 A/cm2 at the specimen surface. The probability of electrons to enter the designed pattern as a function of the defocusing distance are calculated. The best refocus length is obtained to give the maximum probability in the designed pattern at the specimen surface.

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.

Resist Heating in Cell Projection Electron Beam Direct Writing

Electron beam (EB) direct writing systems have often been used for fabricating sub-half-micron advanced devices because EB direct writing is the most practical method for making the required patterns. Recently, the cell projection (CP) method has become indispensable for increasing the writing throughput in the EB direct writing system. However, it is considered that resist heating may be seriously aggravated below the quarter-micron level when the CP method is used, because the total deposited energy, which is irradiated by one CP EB shot, is almost the same as that irradiated by one variably shaped (VS) EB maximum size shot. Resist heating in the case of the CP method is calculated by a finite element method using the ANSYS (Ver. 5.0A: ANSYS, Inc.) program. In particular, thermal diffusion calculation is mainly carried out under the conditions of 50 kV acceleration voltage and 10 A/cm2 current density for practical application to advanced device fabrication. The calculated results suggest that resist heating in the CP method is mainly caused by the horizontal thermal flux between plural EB shots within the area of one CP shot, by the same mechanism as proximity resist heating under the VS method. Therefore, CP EB writing causes horizontal-mode resist heating. In particular, when a low current density is used, this resist heating mode arises significantly. However, CP writing with high acceleration voltage causes a reduction in the rise of the resist temperature, which causes resist heating. When the EB irradiation time is longer than 1.0 µ s under practical EB writing conditions, the resist temperature increases proportionally to the decrease of writing pattern size in the case of the CP writing with a maximum shot size of 5.0×5.0 µ m. It is also shown that the larger the beam blur of an incident beam, the more serious is the resist heating. When a highly sensitive resist (10 µ C/cm2) is used under these practical conditions, however, resist heating in the CP method is prevented without writing throughput degradation regardless of the CP maximum shot size, because the resist temperature does not rise above the thermal denaturation temperature of standard EB resists. Accordingly, the maximum CP shot size, which affects the writing throughput, is determined by the proximity effect and the Coulomb interaction for fine pattern fabrication.

Recent Progress in Electron-Beam Cell Projection Technology

NEC's recent progress in the development of electron-beam (EB) cell projection technology is reviewed. To make it practical, not only the development of a high-performance EB direct writing system but also the establishment of its peripheral technologies in the micro-fabrication process are pursued. In order to obtain high lithographic performance in the EB cell projection lithography system HL-800D (Hitachi), the fundamental effects in EB lithography such as the Coulomb interaction effects, the proximity effect and electron scattering by cell projection aperture (EB mask), have been studied. In addition, high-resolution and high-sensitivity resists have been developed. The Coulomb interaction effects are found to be the most critical issue in cell projection lithography because it affects resolution, linewidth accuracy and throughput. For high resolution, the beam current was reduced to suppress the Coulomb interaction. As a result, a resolution of 0.15 µ m, which is sufficient for fabricating a 1G DRAM, was obtained using the high-resolution resist. To achieve high-linewidth accuracy of less than ±5% for 0.2 µ m lines-and-lines (L/S), optimization of the EB mask structure and the development of a proximity effect correction method which includes the Coulomb interaction effect correction were carried out. Inspection technology for devices of 0.2 µ m or less was also investigated in order to accurately measure linewidth, and to detect defects, particularly those caused by shot stitching error. Finally, the cell projection technology has been applied to the device fabrication of a 1G DRAM, and was demonstrated to be feasible for the development of futuristic advanced devices.

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.

Improved Proximity Effect Correction Technique Suitable for Cell Projection Electron Beam Direct Writing System

Electron beam (EB) direct writing is expected to play an important role in the field of lithography for manufacturing future advanced ULSIs. Cell projection techniques are of particular interest, which may make the EB direct writing system practical for use in ULSI memories with many repeated patterns. However, the proximity effect of such systems disturbs the formation of fine 0.2 µ m level patterns throughout the patterning area. In order to solve this problem, we have developed an improved proximity effect correction technique suitable for use in a cell projection EB direct writing system. This technique makes use of smaller cell projection shot (CPS) size in the edge region than in the center region in the EB direct writing area with dose compensation between CPSs, based on the self-consistent method. Utilizing this technique, we can obtain fine 0.2 µ m patterns with 0.02 µ m critical dimension (CD) control patterns, which are required to manufacture 1Gbit dynamic random access memories (DRAMs).

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.

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.