Ion implantation is a standard technique for introducing conductivity-altering impurities into semiconductor wafers. In a conventional beamline ion implantation system, a desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. Energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Beamline ion implanters are typically designed for efficient operation at relatively high implant energies and may not function efficiently at the low energies required for shallow junction implantation.
Plasma doping systems have been studied for forming shallow junctions in semiconductor wafers. In a plasma doping system, a semiconductor wafer is placed on a conductive platen, which functions as a cathode and is located in a process chamber. An ionizable process gas containing the desired dopant material is introduced into the chamber, and a voltage pulse is applied between the platen and an anode or the chamber walls, causing formation of a plasma having a plasma sheath in the vicinity of the wafer. The applied pulse causes ions in the plasma to cross the plasma sheath and to be implanted into the wafer. The depth of implantation is related to the voltage applied between the wafer and the anode. Very low implant energies can be achieved. Plasma doping systems are described, for example, in U.S. Pat. No. 5,354,381, issued Oct. 11, 1994 to Sheng; U.S. Pat. No. 6,020,592, issued Feb. 1, 2000 to Liebert et al.; and U.S. Pat. No. 6,182,604, issued Feb. 6, 2001 to Goeckner et al.
In the plasma doping systems described above, the applied voltage pulse generates a plasma and accelerates positive ions from the plasma toward the wafer. In other types of plasma systems, known as plasma immersion systems, continuous or pulsed RF energy is applied to the process chamber, thus producing a continuous or pulsed plasma. At intervals, negative voltage pulses, which may be synchronized with the RF pulses, are applied between the platen and the anode, causing positive ions in the plasma to be accelerated toward the wafer.
A surface to be implanted may interact with the ions being implanted in an undesired way. For example, ion implantation may produce charging of insulating or semi-insulating structures on the surface of a substrate. Films or layers, such as photoresist masks, on the substrate surface may release gases and change composition during implantation. The photoresist may be an insulator at the start of an implant and may become more conductive as the implant progresses. These effects may cause unstable and/or non-repeatable implant conditions.
Prior art approaches to dealing with these issues in plasma ion implantation include pretreating the photoresist with ultraviolet light or baking to reduce outgassing. Also, the photoresist can be pretreated by plasma ion implantation of an inert ion species or by plasma immersion, where the substrate is biased positive to extract electrons from the plasma and these extracted electrons pretreat the photoresist. These approaches require an additional process step prior to ion implantation of the dopant material and thus reduce throughput.
Beamline ion implantation systems have used lower initial beam current to reduce photoresist effects. This approach applies to beamline systems and has a disadvantage of changing the spatial charge distribution of the beam and thus impacting the implant uniformity and the implant defect generation that may be dependent upon instantaneous dose rate. The beamline approach can also lead to charge neutralization difficulties, as neutralization systems such as an electron flood gun may be optimized for a specific beam current condition.