Stuart T. Stanton

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...

Finite element analysis of SCALPEL wafer heating

A high-throughput scattering with angular limitation projection electron beam lithography (SCALPEL) tool will typically deliver up to 1.5×10−4 J to an area of 250 μm×250 μm over a time of 200 μs, corresponding to a power input of 0.75 W. This heat deposition occurs in the upper 60 μm of a wafer creating local thermal strain at the time of image formation, and depends on mask and tool conditions and specific boundary conditions. Initial modeling results indicate expansion-induced peak pattern placement errors of ∼1 nm on a local scale (i.e., a 3 mm effective field size), several nanometers in a chip area, and a few microns overall. Since the use of a mainly predictive correction algorithm is anticipated, it is vital to conduct a rigorous investigation of the heat transfer and strain formation. Results are presented of such an analysis, illustrating the heating response for the fundamental building blocks of the SCALPEL writing strategy.

Initial wafer heating analysis for a SCALPEL lithography system

A high-throughput SCALPEL tool will employ a typical exposure current of 30 μA and electron column potential of 100 KV, delivering power up to ∼3 W through a 0.25 mm (wafer scale) square optical sub-field. Electrons lose energy to form heat in the upper 60 μm of a wafer in vacuum during a sub-field exposure period of ∼200 microseconds, creating significant local wafer heating at the time of image formation. Our initial analysis indicates that expansion-induced pattern placement errors will require a sub-field position correction strategy.

Electron-optics method for high-throughput in a SCALPEL system: preliminary analysis

Abstract A likely technology to supplant optical tools for the manufacturing of sub-0.13 μm design rule ICs is one based upon SCALPEL ® (SCattering with Angular Limitation Projection Electron-beam Lithography). One serious barrier to the acceptance of any lithographic technique by the IC manufacturing community is an inability to provide economically viable wafer throughput levels. Using a simple, parametric, time-utilization model of a step-and-scan writing strategy, we have identified the areas of greatest influence on throughput in a SCALPEL system. Though issues such as stage speed, resist sensitivity, and space charge-limited beam current do constrain the problem, we have found that the effective size of the printing field is the most sensitive parameter for realizing high throughput levels in SCALPEL. In this paper we present an electron-optical method for attaining high-throughput in a SCALPEL-based exposure tool. Starting with a moderately large area beam (1 mm × 1 mm) at the mask plane and simple, telecentric reduction (4x) optics, we have investigated increasing the effective printed field size through a combination of beam deflections, image stitching, and dynamic corrections. A preliminary analysis of recent modeling results indicates that a 3 mm × 3 mm effective field size at the wafer can be achieved while maintaining beam blur within manageable limits. The extensibility of this electron-optical approach to a production-worthy level of wafer throughput is presented, including the potential impact on other system parameters.

Critical issues for developing a high-throughput SCALPEL system for sub-0.18-um lithography generations

The potential for SCALPEL to provide economically viable production lithography capabilities for post-optical generations depends largely on achieving adequate wafer throughput. We have analyzed throughput-limiting performance attributes of the SCALPEL approach in order to identify critical design issues and develop a process for evaluating its unique parameter space. An important feature of the SCALPEL approach is that small image sub-fields are assembled to form complete device patterns. Further, electron-electron interactions result in a throughput- dependent image blur, which is a governing parameter for many inter-related performance areas of SCALPEL. Error budgets for key issues affecting critical dimension (CD) have been developed to analyze this unique design space, using models of the image-forming process including stitching on sub-field seams. These budgets assist in identifying the most critical design issues and demonstrating their inter-relationships and tradeoffs.

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.

SCALPEL aerial image monitoring: Principles and application to space charge

We present an approach to real time direct aerial image monitoring which utilizes the information contained in an alignment signal generated by scanning the image of a mask grating over a corresponding wafer grating and detecting the backscatter electron signal. The basic principles of this measurement technique are described. The effect of noise and other common errors, such as magnification and rotation on the signal quality, are derived and used to set requirements on signal contrast and noise level for obtaining blur values accurate to approximately 1 nm. The application of this approach to measuring space charge blur is described and preliminary data illustrating the concept is presented. The potential for this technique to form the basis of an automated self-calibration system on SCALPEL tools is clear.

Overlay error budgets for a high-throughput SCALPEL system

The implementation of SCALPEL for post-optical production lithography generations, including mix-and-match options, involves unique issues in alignment and overlay. SCALPELs use of stitching modifies the familiar analysis of overlay errors. Stitching may produce a small, localized image- placement error, but it creates negligible fixed image distortion. It also allows sub-field placement adjustments to correct some of the distortion errors in mix-and-match optimization. SCALPEL can use existing off-axis alignment sensor technologies, but a preferred electron back-scatter technique offers robustness and versatility. For high- throughput operation, a form of global alignment similar to that of full-field tools is likely, but implemented with the dynamic alignment mark scanning capabilities available in the writing strategy. Finally, it is expected that wafer- heating correction issues will factor into the coupled development of optimum writing and alignment strategies, possibly introducing novel mixed operating modes of fine alignment. We shall discus our present overlay error budgets, representing these unique challenges and opportunities for developing a high-throughput SCALPEL tool.

Critical dimension control at stitched subfield boundaries in a high-throughput SCALPEL® system

Scattering with angular limitation projection electron-beam lithography (SCALPEL®) is a stitching lithography system, using a segmented mask pattern which is assembled on the wafer by dynamic imaging. Critical objects may lie on the original pattern segment boundaries (seams), but their stitched images must adhere to conservatively interpreted +/−10% critical dimension (CD) control requirements. Stitching with butted (touching) spatial image parts requires an aggressive placement-error allocation of approximately +/−CD/20, along with allowed values for dose control, mask, resist, and process in the CD control budget. Further analysis has shown that a seam-blending scheme increases the stitching-error tolerance by as much as five times, and allows redistribution of all values in the CD error budget. The seam-blending approach provides overlapped image portions in a small area common to neighboring segments which receive complementary illumination dose portions.

Analytical-based solutions for SCALPEL wafer heating

When operating in high-throughput conditions, SCALPELs 100 keV electron beam causes significant dynamic wafer deformation. To complement extensive finite element analysis, we have undertaken an analytical solution to the governing heat transfer and elastic strain equations and boundary conditions. This efficient, Green’s function-based computational approach is able to support a robust and versatile image-placement correction strategy.