Electron Beam Projection Lithography System Research Articles

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.

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.

Electron projection lithography tool development status

In the development of an electron beam projection lithography (EPL) tool, the most important tasks are to develop the high-speed vacuum stage system, reliable vacuum body system, and total control system. Nikon has a long history of over 22 years in precision stage development for its optical lithography tools as well as over 10 years in electron beam (EB) instrument development such as the EB 60 with NTT. Recently, optical lithography stages have been developed based on air bearing and linear motor technologies. It is desirable and of minimum risk to utilize those technologies for the EPL system in order to shorten the total time period of development, but the requirements for the EB stage, body, and system control are much different from their optical counterparts and much more difficult. In this article development and implementation of the EPL vacuum stage system, vacuum body system, vacuum loader system, and control system are discussed and overviewed.

Nikon electron beam projection lithography tool development summary

In the development of the electron beam projection lithography (EPL) tool, one of the most important tasks is to develop the high-speed vacuum stage system and reliable vacuum body system. Nikon has a long history of over 22 years in precision stage development for its optical lithography tools as well as over 10 years in electron beam (EB) instrument development such as EB 60 with NTT. Recently, lithography stages have been developed based on air bearing and linear motor technologies. It is desirable and of minimum risk to utilize those technologies for the EPL system in order to shorten total time period of development, but the requirements for the EB stage and body are much different from their optical counterparts and much more difficult. In this paper, development and implementation of the EPL vacuum stage system, vacuum body system, vacuum loader system, and control system are discussed and overviewed.

Fast and Simplified Technique of Proximity Effect Correction for Ultra Large Scale Integrated Circuit Patterns in Electron-Beam Projection Lithography

The pattern shape modification method is an effective proximity effect correction technique for electron-beam projection lithography (EPL) systems. We used two characteristics based on the large difference between the forward and backscattering ranges of incident electrons from the 100 kV EPL system to simplify and speed up the pattern shape modification method. The first characteristic is that the amount of pattern bias becomes almost identical on opposite sides of a sufficiently small pattern compared to the backscattering range. The second characteristic is that although the equations for calculating the amount of pattern bias are simultaneous equations, they can be solved separately if the pattern is sufficiently long compared to the forward-scattering range. In addition, multithreading was used to perform parallel processing for the purpose of speeding up the pattern shape modification method. This simplification and speeding up cut the processing time to 1/3 when using four processors. We also checked the correction accuracy by repetition, and considered the use of a correction that maintains the hierarchy for compressing the amount of data.

Electron Beam Sources for Semiconductor Lithography System.

This paper describes several kinds of electron beam sources and their gun structures for a semiconductorlithography system. In general, high brightness LaB6 thermal-emission type is very common due to the lowworkfunction advantage. However, for an electron-beam projection lithography system (EPL), the high emittance (low brightness) gun should be required in order to achieve excellent illumination uniformity even in theone large exposure field. Furthermore, the direct-bombardment Ta cathode and a unique illumination opticsare needed. They result in excellent pattern line-width fidelity. For the future node devices, a mask-less lithographysystem is also expected. For this purpose, various kinds of multiple beam sources are described. Insummary, optimized electron sources and gun structures should be required along with the suitable electronbeam lithography for every node device.

EB Stepper-A High Throughput Electron-Beam Projection Lithography System

The development program of an electron-beam (EB) stepper is under way based on the results of proof of the concept system. Very careful system consideration has been made, because some critical issues have not yet been proven. However, they could be well managed by implementing a new methodology that is introduced by examining each issue from the viewpoint of the basic electron optics principle. In this paper, the practicability and extendability of an EB stepper are discussed and overviewed.

Electron optical image correction subsystem in electron beam projection lithography

To obtain ultimate image fidelity in the PREVAIL electron beam projection lithography system (EB stepper) [Pfeiffer et al., J. Vac. Sci. Technol. B 17, 2840 (1999)], precise dynamic corrections [Zhu et al., Proc. SPIE 2522, 66 (1995)] of an exposed reticle subfield image on the wafer are required. The electron beam column for the EB stepper covers a large deflection area by a curvilinear variable axis lens (CVAL) [Stickel and Langner, J. Vac. Sci. Technol. B 17, 2847 (1999)] type deflection with high current, and very large 0.25 mm square beam. Because of these features together with tighter specifications for electron optics in below 70 nm node, aberrations which can be negligible in prior art electron beam lithography systems, can no longer be ignored. Therefore the electron optical image correction subsystem in the EB stepper is required to precisely correct the increased numbers of possible measurable aberrations caused by deflection as well as by space charge effects. In this article, a systematic overview of beam corrections is described.

Stencil reticle repair for electron beam projection lithography

Repair of stencil reticles for electron beam projection lithography system is one of the critical issues on reticle manufacturing. Focused ion beam deposition is studied as the method for repairing the clear defects of stencil reticle. The film deposition of diamondlike carbon (DLC) across the stencil pattern, on the sidewall of the stencil, and on the pre-etched slot pattern is demonstrated. The deposited DLC films have good properties as the repair material. Deposited patterns across the stencil pattern are imaged on the resist with the Nikon 100 kV experimental projection column. When the thickness of the deposited DLC film is more than 0.5 μm and the contrast aperture size of the projection column is 1.5 mrad, the thickness of the deposited pattern does not affect the critical dimension of the resist pattern imaged the repaired patterns. The profiles, the pattern size, and the electron scattering properties of DLC films are stable for 100 kV electron beam continuous irradiation (2 C/cm2 dosage; corresponding to half-year dosage on electron beam stepper). Moreover, the repaired pattern is not damaged by the wet megasonic cleaning.

Hollow Beam Electron Gun for Electron Beam Reducing Image Projection System: Computer Simulation

A new hollow beam electron gun with an annular emission area is described. A cathode image is focused at the contrast aperture of the projection lens system, and the mask is irradiated by a uniform intensity electron beam which is conjugate to a flattop region near the crossover. The emittance is 7.8 mm mrad for a beam energy of 100 keV. Brightness can be varied from 8×102 to 4.5×103 A/cm2sr by changing only the cathode temperature. The gun crossover, the flattop area and the cathode image axial positions can also be varied widely, without changing gun brightness and emittance, by changing the gun control anode potential. The emission current can be reduced by reducing the “ratio (emission area radial width/maximum emission area radius) of the cathode surface” without changing the emittance and with little brightness change. Using this simulated electron gun, the basic electron optics of an electron beam projection lithography system with a hollow beam is designed.

High emittance source for the PREVAIL projection lithography system

In the PREVAIL proof of concept electron beam projection lithography system, 1 mm×1 mm sq reticle subfields are illuminated and imaged to a wafer with 4× demagnification. A relatively large beam semiangle (5–8 mr at the wafer) is required to optimize resolution at the beam currents (5–15 μA) needed for high-throughput lithography. A high emittance source and illumination system have been developed which can uniformly illuminate the reticle subfield with a beam semiangle up to 16.3 mr (1/e) at the wafer. The source utilizes a tantalum single crystal 10 mm in diameter. The crystal is heated by electron bombardment incident on the side opposite the emitting surface, which is a low work function crystal plane. A variation of the “critical Köhler” illumination scheme is utilized wherein the cathode surface is imaged approximately to a square shaping aperture, and the shaping aperture is conjugate to the reticle (and the wafer). The emission is temperature limited, so care must be taken to obtain a uniform temperature distribution across the emitter surface. Long term emission current stability better than 1% is provided by servo control of the bombardment heating power. Illumination uniformity has been measured at the wafer plane using a pinhole detector. The measured 3σ variation in current density is 3.2%. On the basis of these results, the tighter specifications required for a commercial PREVAIL projection system appear achievable.