Global space charge effects in ion-beam lithography

Global space charge effects in ion beam lithographyJohn J. Petillo and Alfred A. MondelliApplied Physics Operation, Science Applications International CorporationMcLean, VA 22102ABSTRACTIon Projection Lithography (IPL) is a candidate technology for meeting expected circuit design rules (less than 0.18micrometers) for future generations of semiconductor devices. The Advanced Lithography Group (ALG), a consortium ofindustrial and US government laboratories and universities, is developing IPL technology with funding from ARPA and thestate of Maryland. A prototype IPL device, the ALG-1000, is being developed to demonstrate the capability of IPL to meetfuture requirements for pattern overlay.To realize IPL technology requires control of space charge effects in the ion optical column. Due to the length of the IPLsystem (several meters), the precision of the calculation to predict distortion at the wafer plane becomes difficult to perform.Including the effects of space charge effects is even more difficult. Both global and stochastic space charge phenomenaoccur. This paper presents a system of computer models that allows simulations of both global and stochastic space chargeeffects. In particular, the models will be used on the ion beam projector in the ALG-1000 device.The calculations are carried out using a self-consistent equilibrium ray tracing code, where the applied fields are calculatedfrom the actual lens column geometry. For global space-charge, the model also includes optimization of the lens electrodevoltages to minimize pattern distortion at the wafer plane.Keywords: global space charge effects, ion beam lithography.1. INTRODUCTIONIon-projection lithography (IPL) offers an attractive solution to future lithographic needs of the semiconductor industry. IPLis being developed by the Advanced Lithography Group, a consortium of industrial and government laboratories anduniversities, that is demonstrating the technical viability of IPL and establishing a commercial infrastructure to support themanufacture of IPL systems. Commercial IPL systems are expected to be available for 0.18, 0.13, and 0.10 jim feature sizesat a cost of ownership comparable to that of competing technology.The viability of IPL for meeting future chip design rules depends on the successful resolution of several technical issues thatare being addressed by the ALG. These include control of global and stochastic space-charge effects in the ion column at thedesign beam current. If these effects are too large, the current would have to be reduced and throughput would suffer. Aprogram of theoretical and experimental studies is being carried out by the ALG to assess space-charge effects.Global space charge effects are those associated with the volume charge density of the ions. As ions propagate within thebeam, they make random Coulomb collisions that produce a stochastic component to the space-charge force. While theglobal space-charge field acts as a nonlinear lens, which in principle can be compensated by other focusing elements, thestochastic space-charge effects produce random displacements, or "blur," in the ion-beam pattern.Experiments on ion-projector systems available to the ALG in Europe have investigated stochastic space charge effects onpattern resolution for beam currents in the range of 1 nA to 2 ptA. For low beam energy (5-7 keV) at the cross-over regionof the projection optics there is a large space charge effect in the system, and the dependence of the stochastic space-charge-induced pattern blur on beam energy and beam current can be confirmed. At higher cross-over energy (50 keY) there is noclear evidence of a loss of pattern resolution with increasing ion current.These results are in general agreement with theoretical models"2 that indicate that the blur due to stochastic space charge willscale as