Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
1999-05-28
2002-07-23
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C315S505000
Reexamination Certificate
active
06423976
ABSTRACT:
FIELD OF THE INVENTION
The invention is concerned with ion implanters and with a method of ion implantation.
BACKGROUND OF THE INVENTION
Ion implanters have been used for many years in the processing of semiconductor wafers. Typically, a beam of ions of a required species is produced and directed at a wafer or other semiconductor substrate, so that ions become implanted under the surface of the wafer. Implantation is typically used for producing regions in the semiconductor wafer of altered conductivity state, by implanting in the wafer ions of a required dopant. Typical ionic species used for this purpose are boron, phosphorous, arsenic and antimony. However, other ionic species are also used for other purposes, including oxygen for example.
The depth to which implanted ions penetrate the surface of the wafer is largely dependent on the energy of the ions in the ion beam. The semiconductor industry requires both very shallow implants, for example for very fine structures having a small feature size, and relatively deep implants, for example for buried layers etc. It is also a general requirement of the semiconductor processing industry that process times should be as short as possible which implies that the quantity of ions being implanted per unit area and time into a semiconductor wafer should be as high as possible. This implies that ion implantation is conducted with a high beam current, being a measure of the number of required ions in the beam reaching the wafer surface per unit time.
Beam energies up to about 200 keV (for singly charged ions) can quite readily be obtained using electrostatic acceleration systems, in which the source of ions is held at a fixed voltage relative to the wafer to be implanted, the fixed voltage defining the energy of the ions in the beam on implantation.
It has been recognized that radio frequency linear accelerators are useful to achieve higher beam energies.
A linear accelerator structure accelerates charged particles of a specific mass/charge ratio which are injected into the accelerator at a specific injection energy. It is the inherent nature of rf linear accelerators that the particles or bunches of particles passing through the accelerator must reach successive accelerating gaps at the right region of the sinusoidal waveform of the voltage applied to the gaps. Essentially, as each particle (or bunch of particles) crosses an accelerating cavity it will receive a certain amount of energy (increase in speed) dependent on the field across the gap at the specific time. If an accelerator is set up for particles of a particular mass/charge ratio and injection energy, the particles accelerated by a first gap will reach the next accelerating gap just as the field across that gap is optimum to provide further acceleration. It will be understood by people skilled in this art that a particle of the same energy but having a higher mass-to-charge ratio crossing the first gap would travel from the first gap at a lower velocity and so would tend to reach the next gap later in the rf wave form that is applied across that gap. Similarly, a lighter particle crossing the first gap would reach the second gap earlier. The accumulated effect of this over multiple accelerating gaps is that particles of mass-to-charge ratios different from the mass-to-charge ratio for which the accelerator is set up arrive at subsequent accelerating gaps at times when they are not suitably accelerated.
As is well known in the linear accelerator art to produce high energy beams of different ionic species the set up of the accelerator requires change to match the mass-to-charge ratios of the selected ions. Among ions useful for implantation, singly charged boron (B
+
) has a mass/charge ratio of about 11, whereas singly charged phosphorous (P
+
) has a mass/charge ratio of about 31. Singly charged arsenic has a mass-to-charge ratio of about 75 and singly charged antimony has a mass-to-charge ratio of about 122.
The use of rf linear accelerators for ion implantation has been suggested at least since 1976 in “Upgrading of Single Stage Accelerators” by K. Bethge et al, pages 461-468, Proceedings of the Fourth Conference on the Scientific & Industrial Applications of Small Accelerators, North Texas State University, Oct. 27-29, 1976; and in “Heavy Ion Post-acceleration on the Heidelberg MP Tandem Accelerator”, edited by J. P. Wurm, Max Planck Institute for Nuclear Physics, Heidelberg, May 1976. U.S. Pat. No. 4,667,111 discloses an ion implanter incorporating a radio frequency linear accelerator to provide ultimate beam energies as high as 2 meV or more. The rf linear accelerator is formed of a series of so called two gap accelerating cavities. For set up of the accelerator, with the frequency of the rf fields in successive cavities of the accelerator kept the same, the phase of the wave form for one two-gap cavity relative to the preceding two-gap cavity is adjusted so that the correct point of its waveform is presented to arriving ions of the selected species. The resulting two-gap tool tends to be very long relative to the performance achieved; the specification contemplates using ten or more two-gap cavities in succession, and is limited to relatively low beam currents. Whereas a low beam current may be satisfactory at high energies, when the apparatus is operating at relatively lower energies, higher beam currents are desirable to improve the processing speed.
Japanese Patent Application Publication No. Hei 9-237700 (1997) discloses an ion implanter using an rf accelerator formed with one or more three gap rf accelerator cavities. In this context it will be understood by those skilled in the art of linear accelerators that a two gap accelerator cavity, e.g. as used by the above referred U.S. patent, has entrance and exit electrodes at a fixed, usually ground, potential and a single intermediate electrode to which is applied the rf potential, thereby forming a pair of accelerating gaps on opposite sides of the rf electrode. As is also well known in the art, a three gap cavity has entrance and exit electrodes at a fixed, usually ground, potential and a pair of intermediate electrodes defining three gaps. The intermediate electrodes are energised by the rf potential with opposite polarity. Thus, if the amplitude of the energising rf voltage is V, the maximum accelerating potential across the first and last gaps of the cavity is V whereas the maximum accelerating potential between the two intermediate electrodes is 2V.
In the above Japanese publication, the injection energy to the three gap rf accelerator cavity appears to be relatively high. The specification contemplates some form of beam accelerator upstream of the three gap cavity but downstream of the usual analyser magnet, which separates from the ions emitted from an ion source the particular species of ion required for implantation. U.S. Pat. No. 5,801,488, which is assigned to the same Assignee as the above Japanese patent publication, discloses the provision of an rf quadrupole accelerator upstream of the three-gap linear accelerator stages.
Reference may also be made to Japanese Patent Publications Nos. Hei 7-57897 and Hei 7-57898 which disclose features of the same machine, and also to the article “The development of a beamline using an RFQ and three gap rf accelerators for high energy ion implanter”, Fujisawa et el, presented at IIT, Kyoto, Jun. 24th 1998.
Generally, the above Japanese references disclose an implanter tool which is likely to be very large and expensive to build. Furthermore, beam currents when operating at relatively lower energies will be very small.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an ion implanter using at least one rf accelerator stage, which can generate a high energy beam as well as operate at lower energies with good beam current.
Accordingly, in one aspect the invention provides an ion implanter comprising an ion beam generator for generating a beam of ions to be implanted in which said ions are at a first energy, and a radio frequency linear
Glavish Hilton F.
Gordon John Stuart
Applied Materials Inc.
Boult Wade & Tennant
Nguyen Kiet T.
LandOfFree
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