It is known in the prior art to dope semiconductor wafers by causing ions to impact the wafers until a specified concentration of ion doping occurs. Typical ion implant systems used for such doping include an ion source and an ion analyzer for insuring ions of an appropriate mass make up the ion beam striking the wafers. As the ion beam passes from the ion source to the wafers, steps are typically also taken to avoid ion beam degradation.
One such step is an attempt to maintain beam neutrality. As ions travel along a beam path to the wafers, they move through a relatively low-pressure gas. Ions impact gas molecules, ionizing them to provide low-energy electrons. The low energy electrons are trapped by the positive space charge potential of the ion beam and partially neutralize the beam, decreasing its space charge divergence. Severe space charge divergence can induce loss in beam transmission and have other negative effects on beam performance.
At the region of the target wafer the value of the beam potential depends upon the impedance to ground of the wafer. If the impedance is large (i.e., the wafer is floating), at equilibrium the wafer potential is equal to the beam potential measured anywhere along the ion beam, so long as the ion beam is isolated from acceleration electrodes used to add energy to the ion beam.
Large ion beam potentials cause the wafers to charge during ion implantation. To increase beam neutralization and hence reduce wafer charging, it is possible to increase the concentration of low-energy electrons. A number of electron concentration control schemes have been devised and currently used with various degrees of success. (10 volts is a typical ion beam potential at the region of the wafer).
Charging of insulated surfaces on the wafer during implantations can result in dielectric breakdown. Gate oxides in high-density devices can be as thin as 10 nm, and although the dielectric strength of SiO.sub.2 is quite high (10 megavolt/cm) at these kinds of thicknesses, dielectric breakdown can easily occur.
In high current implanters, typically wafers are placed on a support disk which rotates through a fixed ion beam. The ion beam alternatively impacts the wafers, which are insulated from ground or it impacts the support disk, which is at ground potential. The beam potential at the implantation location is constantly varying and may be an important cause of ion dose non-uniformity.
The ionization of the residual gas along the beam travel path, in addition to creating electrons, also produces an equal amount of low energy gas ions. These slow ions accelerate radially out from the beam due to the space charge of the ion beam and impact the chamber walls that bound the beam path.
One can estimate the level of low-energy ion currents to be expected due to ion interaction with the gas molecules. The scattering rate for ion interaction with a gas molecule is given by r=n.sigma.dI, where n is the gas density, .sigma. the ionization and charge exchange cross sections, d is the beam path length, and I is the beam current. Typically .sigma.=10.sup.-15 cm.sup.2, at a pressure of 10.sup.-5 torr, for d=1 cm, one finds r=2.times.10.sup.-4 *I. For a 5 mA ion beam, this results in total energy ion currents of the order of one microamp. The actual low energy ion current that is measured depends on details of the implanter geometry and will also depend upon the beam potential (V) itself.