Pressure measurement repeatability in high current ion implanters using hot cathode ionization gauge with design and operation optimized for stability

Abstract

Long-term pressure measurement repeatability in high current ion implanters is an important factor for ion beam current measurement and repeatable dose delivery. In the Axcelis GSD® end station, the issue is particularly acute owing to the small volume of the process chamber. High pressure conditions arise during ion beam irradiation of photoresist layers which, in turn, leads to significant ion beam current neutralization. The GSD design employs a “pressure compensation” algorithm (PCOMP) to offset potential ion beam current measurement errors; the success of this technique depends entirely on accurate and repeatable ion gauge pressure measurement. Incorrect pressure measurement directly leads to dose errors. For example, the dose equation using PCOMP tells us that for a modest PCOMP value of 30%, a chamber pressure measurement error of 2.0E−5Torr can result in a dose error of 5% at normal process pressures. Standard hot cathode ion gauges used for pressure measurement are not capable of meeting the requirements for good dose control since gauge to gauge differences are not well controlled and gauge accuracy is only on the order of 30%. Of more consequence is the degradation of gauge response over time. Axcelis introduced the Stabil-Ion® Bayard-Alpert (BA) gauge to improve dose reproducibility through much improved gauge to gauge matching (±6%) and more accurate gauge output. This article discusses the operation of this gauge system in a production environment and its impact on process dose and process trends. The focus of the discussion is directed to a particular ion implanter design (Axcelis GSD®) since it is unique in its dependence on a PCOMP scheme to counter the effects of ion beam neutralization. However, the above gauge or any similarly stabilized BA gauge would be of benefit to the long-term output and process trends of most ion implant systems.