Electric lamp and discharge devices – With positive or negative ion acceleration
Reexamination Certificate
1999-06-07
2002-02-12
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With positive or negative ion acceleration
C313S230000, C313S360100, C313S361100, C313S362100, C313S363100, C315S111810, C315S111410
Reexamination Certificate
active
06346768
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to an apparatus for generating a beam of charged particles, particularly to an ion gun for use in an ion beam processing apparatus and to an ion beam processing apparatus incorporating same.
Ion beams have been used for many years in the production of components in the micro-electronics industry and magnetic thin film devices in the storage media industry. Typically, an ion beam, such as an argon ion beam, is required to have a large area, a high current and an energy of between 100 eV and 2 keV. The beam can be used in a number of ways to modify the surface of a substrate, for example by sputter deposition, sputter etching, milling, or implantation.
In a typical ion beam source (or ion gun) a plasma is produced by admitting a gas or vapour to a low pressure discharge chamber containing a heated cathode and an anode which serves to remove electrons from the plasma and to give a surplus of positively charged ions which pass through a screen grid or grids into a target chamber which is pumped to a lower pressure than the discharge chamber. Ions are formed in the discharge chamber by electron impact ionisation and move within the body of the ion gun by random thermal motion. The plasma will thus exhibit positive plasma potential which is higher than the potential of any surface with which it comes into contact. Various arrangements of grids can be used, the potentials of which are individually controlled. In a multigrid system the first grid encountered by the ions is usually positively biased whilst the second grid is negatively biased. A further grid may be used to decelerate the ions emerging from the ion source so as to provide a collimated beam of ions having more or less uniform energy. For ion sputtering a target is placed in the target chamber where this can be struck by the beam of ions, usually at an oblique angle, and the substrate on to which material is to be sputtered is placed in a position where sputtered material can impinge on it. When sputter etching, milling or implantation is to be practised the substrate is placed in the path of the ion beam.
Hence, in a typical ion gun an ion arriving at a multiaperture extraction grid assembly first meets a positively biased grid. Associated with the grid is a plasma sheath. Across this sheath is dropped the potential difference between the plasma and the grid. This accelerating potential will attract ions in the sheath region to the first grid. Any ion moving through an aperture in this first grid, and entering the space between the first, positively biased grid, and the second, negatively biased, grid is strongly accelerated in an intense electrical field. As the ion passes through the aperture in the second grid and is in flight to the earthed target it is moving through a decelerating field. The ion then arrives at an earthed target with an energy equal to the potential of the first, positive, grid plus the sheath potential.
Hence, a conventional ion gun comprises a source of charged particles which are accelerated through an externally applied electric field created between a pair of grids. Conventionally, for low energy ion beam production, three grids are used, the first being held at a positive potential, the second being held at a negative potential adjusted to give the best divergence, and the third, if present, at earth potential, i.e. the potential of the chamber in which the beam is produced. Beams of this nature are well described in the open literature going back over 25 years.
In some applications it is desirable to obtain an ion beam of maximum current. However, in other applications it is the divergence of the ions comprising the beam, or rather the relative lack of it, that is critical to achieving a suitable process performance.
U.S. Pat. No. 4,447,773 discloses an ion beam accelerator system for extracting and accelerating ions from a source. The system includes a pair of spaced, parallel extraction grids, 60 mil (1.524 mm) apart, having aligned pairs of holes for extracting ion beamlets. The pairs of holes are positioned so that the beamlets converge and the merged beamlets are accelerated by an accelerator electrode which is 0.6 inch (15.24 mm) downstream of the extraction grid pair. The extraction grids are formed with numerous small holes through which beamlets of ions can pass and are maintained a potential difference of a few hundred volts. The accelerator electrode has a single hole, which is slightly greater in height than the height of the matrix of holes in the extraction grids, and is maintained at a much lower potential for accelerating the converged ion beam emerging from the extraction grid pair.
An extensive introduction to and prior art review of ion beam technology is provided in EP-B-0462165. EP-B-0462165 itself describes an ion gun in which the plasma from which the ions are accelerated by the accelerator grid is at a low potential of not more than about 500 V and is of uniform density so as to permit high current densities of the order 2 to 5 mA/cm
2
in the ion beam to be achieved at low potential (i.e. less than about 500 V), and with minimum risk of damage to the accelerator grid, in operation. This system provides an ion gun in which the plasma can be efficiently generated using a commercially acceptable high radio frequency, such as 13.56 MHz or a multiple thereof, and in which the resulting plasma has the desirable properties of high density, good uniformity and a relatively low plasma potential.
However, a perennial problem with the ion beam sources described in the prior art is the high magnitude of divergence to which the beam is susceptible. Whilst the system of EP-B-0462165 solves many of the problems associated with other prior art ion beam sources, it would still be highly desirable to improve this system to provide a reduced degree of ion beam divergence. There is a growing demand for ultra low divergence at low to medium beam energy. In this role the beam is typically described as an ion mill selectively etching deep trenches of perhaps 1×10 &mgr;m scale length. To do so requires a beam with a divergence of no more than approximately 1° at an energy dictated by the constraints of maximum rate at a processing energy of perhaps as low as 500 eV. Conventional ion guns operating at this energy cannot meet the divergence requirement at a high enough current to meet the process rate.
SUMMARY OF THE INVENTION
The present invention accordingly seeks to provide an ion gun which is capable of operation in a manner such that the above aims are substantially achieved.
According to the present invention there is provided apparatus for the production of low energy charged particle beams comprising: a plasma chamber; means for generating in the plasma chamber a plasma comprising particles of a first polarity and oppositely charged particles of a second polarity; means for restraining particles of the first polarity in the plasma chamber; a first multi-aperture electrode grid contacting the plasma, wherein the first electrode grid is arranged for connection to a first potential source so as to impart to the first electrode grid a first potential of the second polarity; a second multi-aperture electrode grid arranged for connection to a second potential source so as to impart to the second electrode grid a second potential, the second potential being less than in the sense of being either less positive than or less negative than the first potential so as to produce between the first and second electrode grids a first acceleration field for accelerating charged particles of the second polarity towards and through the second grid; and a third multi-aperture electrode grid arranged for connection to a third potential source so as to impart to the third electrode grid a third potential of the first polarity and to produce between the second and third electrode grids a second acceleration field for accelerating charged particles of the second polarity towards and through the third electrode grid, the grid spacing between the first a
Haynes Mack
Nordiko Limited
Patel Vip
Senniger Powers Leavitt & Roedel
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