Victor Katsap

Global space charge effect in SCALPEL

The global space charge (SC) effect in SCALPEL electron beam lithography system is investigated. First order properties of the SC lensing action (defocus and magnification change) in SCALPEL type projection systems are analyzed using a simple analytical technique. Aberrations induced by the lenses and SC in the projection optics are evaluated numerically using a Monte Carlo code developed to calculate the combined effect of Coulomb interactions and lens aberrations in the charge particle projection systems. We found that the defocus and the magnification change induced by SC are functions of two parameters, the beam perveance and the SCALPEL aperture size, that are critical for the system performance. The strong correlation identified between the best focus plane location and the aberrations induced by SC indicates that the SC lensing action can be effectively compensated by simply adjusting either the wafer plane position or excitations of projection lenses.

Scanning thermionic emission imaging of cathode surfaces

The evaluation of the thermionic emission characteristics of a variety of cathode technologies that may have application in the SCALPEL (Scattering with Angular Limitation Projection Electron-beam Lithography) instrument, has been conducted in a modified Scanning Electron Microscope. The thermionic emission image of the source, projected along the column axis by the electrostatic immersion lens of the gun, is analyzed by scanning this image across the entrance aperture of the column by driving the normal alignment coils with the scan drive electronics. The technique may be referred to as Scanning Thermionic Emission Electron Microscopy (STEEM). With this technique, the projected image of the cathode, the cross- over(s) and the cathode surface itself may be imaged by adjusting the focal length of the double condenser lens of the SEM. Images of 0.84 mm diameter cathodes in various forms, including polycrystalline Ta, coated polycrystalline cathodes and single crystal Ta have been examined at a range of magnifications. In addition to operation of the gun with a single large diameter Wehnelt aperture (2.5 mm), cathodes have been studied when located behind a fine mesh in the Wehnelt aperture. Multiple cross-overs with associated beams overlapping further along the axis of the instrument are observed. Single crystal Ta cathodes have been shown to provide the extremely uniform emission in the temperature limited mode, that is required for the effective operation of the SCALPEL technique.

Mesh-equipped Wehnelt source for SCALPEL

The SCALPEL E-beam lithography tool requires an extremely uniform, high emittance E-beam to illuminate the Mask. The existing SCALPEL source utilizes a pure metal cathode operating in the temperature limited mode, thus having limited total emission current available. The usable emitter size of this cathode is constrained by its direct heating scheme, which sets an upper limit for the beam emittance. Furthermore, to generate a uniform beam, a conventional source should be designed to have a high cut-off voltage, which precludes an efficient electronic beam current control. We have studied the possibility of implementing a large area flat cathode and fine control grid mesh combination, potentially capable of providing high emission uniformity, high beam current and low beam control voltage.

3D simulation of flat-cathode electron guns with space charge

Simulation software has been written to calculate the properties of electron guns which have flat cathodes and a fully three-dimensional electrode structure. Volumetric space charge effects are significant in such guns and a three- dimensional solution of Poissons equation is required. This, in turn, requires a simulation of the cathode and its emission properties, a three-dimensional electrostatic potential calculation and a direct ray trace of charged particles through the gun. The motivation for this work was to have software tools to enable the analysis and design of flat cathode guns, for example, guns for cathode ray tubes (CRTs) and a suitable source for the SCALPEL projection lithography column. CRT guns usually have three cathodes and non-circular apertures in the electrodes. A proposed design for the SCALPEL gun has a large-area flat cathode with a fine grid parallel to the cathode and just in front of it. The techniques used in the software are presented and described and the capabilities of the software are illustrated by the analysis of some flat cathode guns.

Uniform large-area thermionic cathode for SCALPEL

An electron beam lithography tool, which employs the SCALPEL technique, requires an extremely uniform beam to illuminate the scattering Mask, with the cathode operating in the temperature limited mode. It has been previously shown that LaB6 cathodes are not stable in this mode of operation. We have explored the possibility of implementing refined Tantalum-based emitters in the SCALPEL source cathode, and have developed large-area flat cathodes featuring suitably high emission uniformity under temperature limited operation.

LaB6 cathode workfunction evaluation

LaB6 cathode is the emitter of choice in electron beam lithography tools. In commercial LaB6 cathodes, the (100) crystalline plane is used as the emissive surface. Values of (100) and surrounding crystalline planes workfunction (WF) vary widely in literature. We have devised and implemented a simple, robust technique for evaluating LaB6 cathode workfunction.

Computing the effects of discrete Coulomb interaction in triode gun with magnetic lens

Boersch, or stochastic space charge effect, is the effect of broadening of the e-beam axial energy spectrum. It limits resolution of fine electron optics by means of change in axial velocity of electrons. This is caused by a stochastic, discrete el ectron-electron interactions in an electron beam. We have computed beam energy spectrum widening in triode gun with magnetic lens.

Grid-controlled 100-kV electron-beam source for SCALPEL [lithography]

Results show that, with the grid-controlled gun, uniform beam profile can be obtained using nonuniform surface cathode. One can conclude that a traditional polished tantalum emitter might be replaced with more efficient, long lasting one in the SCALPEL grid-controlled gun. Both modeling and experimental results show the possibility of building a 100 kV electron beam source providing high-current, high-emittance, uniform electron beam for the high throughput SCALPEL exposure tool.

Ternary ReB6 emitters: A brief study

Binary rare-earth lanthanum hexaborides LaB6 and CeB6 have been successfully used in specialty electron guns, such as e-beam lithography systems, for over two decades. During this time, there were a few papers suggesting ternary compound crystal ReB6 may be a better emitter than binary crystals. We have started studying basic emissive properties of the ternary compound LaxCe1-xB6.

Optics and emission imaging of a LaB 6 emitter

LaB6 cathode is the emitter of choice in electron beam lithography tools. In commercial LaB6 cathodes, the (100) crystalline plane is used as the emissive surface. Typical size of emitter is ~70 μm DIA. Quality control is usually done by observing (100) surface with a powerful microscope. Emission images of the same cathodes often reveal irregularities practically invisible or barely visible neither in optical microscope nor in SEM. We have obtained emission images of the LaB6 cathodes and compared them to optical and SEM images.