Inductively-coupled plasma (ICP) sources have advantages over other types of plasma sources when used with a focusing column to form a focused beam of charged particles, i.e., ions or electrons. The inductively-coupled plasma source is capable of providing charged particles within a narrow energy range, which allows the particles to be focused to a small spot. ICP sources, such as the one described in U.S. Pat. No. 7,241,361, which is assigned to the assignee of the present invention, include a radio frequency (RF) antenna typically wrapped around a ceramic plasma chamber. The RF antenna provides energy to maintain the gas in an ionized state within the chamber.
The energy of ions used for ion beam processes is typically between 5 keV and 50 keV, and most typically about 30 keV. Electron energy varies between about 500 eV to 5 keV for a scanning electron microscope system to several hundred thousand electron volts for a transmission electron microscope system. The sample in a charged particle system is typically maintained at ground potential, with the source maintained at a large electrical potential, either positive or negative, depending on the particles used to form the beam. Thus, the ion beam source is typically maintained at between 5 kV and 50 kV and the electron source is typically maintained at between 500 V and 5 kV. “High voltage” as used herein means positive or negative voltage greater than about 500 V above or below ground potential. For the safety of operating personnel, it is necessary to electrically isolate the high voltage components. The electrical isolation of the high voltage plasma creates several design problems that are difficult to solve in light of other goals for a plasma source design.
One design difficulty occurs because gas must be brought into the high voltage plasma chamber to replenish the gas as ions leave the plasma. The gas is typically stored at ground potential and well above atmospheric pressure. Gas pressure in a plasma chamber typically varies between about 10−3 mbar and about 1 mbar. The electrical potential of the gas must be brought to that of the high voltage plasma and the pressure of the gas must be decreased as the gas moves from the gas source into the plasma chamber. The gas must be brought into the chamber in a way that prevents a gas phase discharge, also known as arcing, which would damage the system.
Another design challenge is to place the radio frequency coils that provide power to the plasma as close as possible to the plasma to efficiently transfer power. Maintaining the coils at the same high potential as the plasma, however, would typically require maintaining the power supply for the coil at the high plasma potential, which would excessively complicate the power supply design and greatly increase the cost. Inductively-coupled plasma ion sources may use a split Faraday shield to reduce capacitive coupling between the coil and the plasma. The split Faraday shield must be located between the plasma and the coils and is typically well grounded. When the grounded Faraday shield is located close to the dielectric plasma container, the large electric field caused by the rapid change in potential would likely cause a gas-phase discharge if any air or other low dielectric constant gas is trapped between the Faraday shield and the dielectric plasma chamber, which discharge could damage the source.
Also, the energy applied to the plasma chamber generates heat. While a compact plasma source is desirable for beam formation, the more compact and powerful the plasma source, the hotter the source would become and therefore the greater the need to efficiently dissipate the heat. The high voltage can also make cooling difficult, which can limit the density of the plasma used. These conflicting requirements make the design of an ICP source very challenging.