James Alexander Liddle

Projection electron‐beam lithography: A new approach

Projection electron‐beam lithography is potentially one of the most attractive techniques available. It offers high resolution, high throughput, and good overlay and registration characteristics. In this paper we discuss some new approaches which seem to offer solutions to problems associated with earlier systems.

Mask fabrication for projection electron‐beam lithography incorporating the SCALPEL technique

The choice of mask materials and fabrication route for a projection electron‐beam lithography system is subject to a variety of constraints. Some are encountered in most lithographic techniques, while some are unique to the scattering with regular limitation for projection electron lithography SCALPEL technique. We have developed methods for analyzing the performance of potential mask materials and constructions. These have been used to determine the composition of a prototype mask. Results from such a mask are presented.

Space charge effects in projection charged particle lithography systems

Charged particle image projection lithography systems have been proposed and are currently under development for design rules of 0.18 μm and below. Although charged particle projection lithography systems do not suffer from diffraction as a limit to spatial resolution as in photolithography, image degradation due to the effects of mutual repulsion of particles in the beam, space charge, will ultimately limit the performance of these systems. Space charge effects increase with increasing beam current. The uncorrectable image blur caused by space charge effectively reduces the dose latitude in projection charged particle lithography and therefore limits the ultimate throughput of such systems. We will describe the effects of space charge in charged particle projection lithography systems using a model we have previously developed. We will compare the predictions of the model with experimental data for an ion projection system and predict the performance of electron and ion beam systems under development. Th...

Electron scattering and transmission through SCALPEL masks

Electron scattering in thin solid films used for the fabrication of masks for electron projection lithography, e.g., SCALPEL®, is investigated. We have developed an analytical model to calculate electron transmission through the mask membrane and image contrast due to different scattering properties of the patterned area and the membrane. The model utilizes cross sections for electron elastic and inelastic scattering on an atom with exponentially screened Coulomb potential of the nucleus derived in the first Born approximation. The variety and controversy of theoretical and empirical adjustments of the screening parameter are briefly analyzed and attributed to the misinterpretation of experimental data ignoring the effects mostly due to plural scattering of electrons and dense packing of atoms in thin solid films. This model frees us from the computational limitations of Monte Carlo simulations and proves to be effective for the straightforward characterization of various alternative materials for SCALPEL...

An analytical model of stochastic interaction effects in projection systems using a nearest‐neighbor approach

Image blurring as a result of stochastic particle interactions has been investigated for projection electron‐ and ion‐beam lithography systems. The investigation was made on the basis of a simple, nearest‐neighbor, analytical model, proposed and developed here, for stochastic particle–particle interactions. The results obtained using this model are in close agreement with those given by Jansen [Coulomb Interactions in Particle Beams (Academic, Boston, 1990)] for an extended parallel beam segment in the Holtsmark regime; at the same time they are extendable over a wide range of conditions, unlike Jansen’s results. The results obtained for a parallel beam are applied to more realistic systems by dividing the beam into nearly cylindrical, uncorrelated slices. This method is used to determine the dependence of the image blur on beam parameters for a doublet. Our results correlate well with those obtained by Monte Carlo calculations.

Particle–particle interaction effects in image projection lithography systems

Using commercially available software (Discrete Coulomb Interactions Software from Munro’s Electron Beam Software Ltd.) we have investigated image broadening as a result of stochastic interactions for projection systems. We have derived empirically, design constraints applicable to ion and electron projection systems and used them to analyze system designs suggested in the literature. We conclude that for many of the suggested designs stochastic interactions will prevent useful throughputs from being achieved. Finally, we discuss system design approaches which are necessary for a successful high‐throughput, high‐resolution image projection lithography system.

Preliminary results from a prototype projection electron‐beam stepper‐scattering with angular limitation projection electron beam lithography proof‐of‐concept system

We have designed and constructed a proof‐of‐concept projection electron beam lithography system based on the scattering with angular limitation projection electron beam lithography principle. In this system, a thin membrane mask is used in a 4:1 reduction projection system at 100 keV. Image contrast is formed by scattering in the mask and subsequent aperturing of the scattered electrons in the back focal plane of the projection system. We have employed a step‐and‐scan architecture which uses continuously moving mask and wafer stages to trace out the full pattern. The electron beam can thus be kept small (1×1 mm in our case) which greatly simplifies the design of the electron optical system. In addition, the membrane areas can be kept small in linear dimension in one direction, minimizing in‐plane pattern distortions. Our system will be constructed in two stages. In the first stage, the mask stage is static and the wafer stage operates in step‐and‐repeat mode. This initial version of the system allows for ...

Marks for alignment and registration in projection electron lithography

This article discusses the relevant criteria for selecting alignment marks for projection electron lithography. The mark material, topography, and pattern layout are considered. Results from experiments and calculations indicate that there is a wide range of acceptable mark configurations suitable for use with short beam dwell times. These results are based on analyses of the available backscattered electron signal and experimentally obtained detection accuracy within the nanometer range.

Space-charge effects in projection electron-beam lithography: Results from the SCALPEL proof-of-lithography system

In projection electron-beam systems resolution and throughput are linked through electron–electron interactions collectively referred to as space-charge effects. Hence, a detailed understanding of these effects is essential to optimizing the lithographic performance of a projection electron-beam lithography system. Although many models have been developed to describe one or more of the various aspects of the Coulomb interactions that occur in the beam, there is minimal experimental data available. We have performed a series of experimental measurements in the scattering with angular limitation projection electron-beam lithography (SCALPEL) proof-of-lithography system to characterize the space-charge effects for such an optical configuration. The results of those measurements have been compared to a combination of computer simulations and analytical models. The agreement between the models and experiments was good, within the limits of experimental error. We determined the exponent in the dependence of blu...

Mark detection for alignment and registration in a high-throughput projection electron lithography tool

The design considerations and performance requirements of an alignment and registration method for high‐throughput projection electron lithography are discussed. We have devised a mark detection system for a lithography tool employing the SCALPEL principle. The process involves scanning the image of a mark from a mask, over a mark that is fabricated on a wafer and measuring the integrated backscattered‐electron intensity. Using a mark that consists of equally spaced lines of tungsten on silicon, we have investigated the effects of spatial and rotational misalignment, and focus in a small field of view SCALPEL machine. We have modeled these and other effects and found agreement between experimental and theoretical results. The measured precision of estimating position from the experimental data is ∼10 nm. By extrapolation we believe that in a practical lithography machine this value can be improved significantly.