Electron beam ion source with integral low-temperature vaporizer

Abstract

An ion source for ion implantation system and a method of ion implantation employs a controlled broad, directional electron beam to ionize process gas or vapor, such as decaborane, within an ionization volume by primary electron impact, in CMOS manufacturing and the like. Isolation of the electron gun for producing the energetic electron beam and of the beam dump to which the energetic beam is directed, as well as use of the thermally conductive members for cooling the ionization chamber and the vaporizer, enable use with large molecular species such as decaborane, and other materials which are unstable with temperature. Electron optics systems, facilitate focusing of electrons from an emitting surface to effectively ionize a desired volume of the gas or vapor that is located adjacent the extraction aperture. The components enable retrofit into ion implanters that have used other types of ion sources. Demountable vaporizers, and numerous other important features, realize economies in construction and operation. Achievement of production-worthy operation in respect of very shallow implants is realized.

Description

[0001] We present the design and operation of an ion source for use in the ion implantation of semiconductors, and for the modification of the surfaces of materials. The ion source can be retrofitted into the existing fleet of ion implanters currently used in the manufacture of semiconductor devices, particularly those used in Complementary Metal-Oxide Semiconductor (CMOS) manufacturing. The ion source is specifically designed to accomodate the use of new solid feed materials such as decaborane (B.sub.10H.sub.14) and Trimethyl Indium (TMI), which vaporize at sufficiently low temperatures that currently available ion implant ion sources cannot use them. Indeed, the currently available ion sources result in dissociation of decaborane when that material is introduced into them. The ion source has an integral low-temperature vaporizer, and a means of introducing the vaporized feed material into an ionization chamber which is also temperature controlled by the vaporizer. The feed materia...

AI Technical Summary

Benefits of technology

[0029] An alternative embodyment of the operation of the vaporizer PID temperature controller is described as follows. In order to establish a more repeatable and stable flow, the vaporizer PID temperature controller receives the output of an ionization-type pressure gauge which is typically located in the source housing of commercial ion implanters to monitor the sub-atmospheric pressure in the source housing. Since the ionization gauge output is proportional to the gas flow into the ion source, it's output can provide an input to the PID temperature controller. The PID temperature controller can subsequently raise or diminish the vaporizer temperature, thus increasing or decreasing gas flow into the source, until the desired gauge pressure is attained. Thus, two complementary operating modes of the PID temperature controller are defined: temperature-based, and pressure-based. These two approaches can be combined so that short-term stability of the flow rate is accomplished by temperature programming alone, while long-term stability of the flow rate is accomplished by adjusting the vaporizer temperature to meet a pressure setpoint. The advantage of such an approach is that, as the solid material in the vaporizer crucible is consumed, the vaporizer temperature can be increased to compensate for the smaller flow rates realized by the reduced surface area of the material presented to the vaporizer.

[0044] In an alternate embodyment of the invention, the electron beam dump is biased to a negative potential relative to the ionization chamber, at approximately the cathode potential, allowing for a "reflex geometry" whereby the primary electrons emitted by the electron gun are reflected from the beam dump back into the ionization chamber and to the cathode, and back again repeatedly. An axial magnetic field may also be established along the direction of the electron beam by the introduction of a pair of Helmholtz coils external to the ion source, to provide confinement of the primary electron beam as it is reflected back and forth between the cathode and beam dump. This feature also provides some confinement for the ions, increasing the efficiency of creating certain desired ion products, for example B.sup.+ from BF.sub.3 feed gas.

Problems solved by technology

A very significant problem which currently exists in the ion implantation of semiconductors is the limitation of ion implantation technology to effectively implant dopant species at low (e.g., sub-keV) energies.

Ion implanters are inefficient at transporting low-energy ion beams due to the space charge within the ion beam causing the beam profile to grow larger (beam blow-up) than the implanter's transport optics, resulting in beam loss through vignetting.

Since this magnetic field also exists in the beam extraction region of the implanter, it deflects the low-energy beam and substantially degrades the emittance properties of the beam, further reducing beam transmission through the implanter.

For example, at 500 eV transport energy, many ion implanters currently in use cannot transport enough boron beam current to be useful in manufacturing; i.e., the wafer throughput is too low.

Also, the vaporizers of current ion sources cannot operate reliably at the low temperatures required for decaborane.

This is due to radiative heating from the hot ion source to the vaporizer causing thermal instability, and the fact that the vaporizer feed lines k, l easily become clogged with decomposed vapor as the decaborane vapor interacts with their hot surfaces.

Hence, the prior art of implanter ion sources is incompatible with decaborane ion implantation.

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