FR2462718A1 - DEVICE FOR MEASURING A IONIC CURRENT ARRIVING IN THE VACUUM ON A MEASURING SURFACE - Google Patents

Abstract

WITH REGARD TO THE MEASURING SURFACE 2 ARRANGED IN A VACUUM ENCLOSURE AND RECEIVING AN INCIDENT IONIC BEAM, AN AUXILIARY ELECTRODE 5 AND MAGNETS 3 ARE MOUNTED TO PRODUCE A TRANSVERSAL MAGNETIC FIELD TO THE DIRECTION OF THE INCIDENT IONIC BEAM. THE MAGNET SYSTEM IS SPACED A DISTANCE A FROM THE MEASURING SURFACE 2, AND THE AUXILIARY ELECTRODE 5 SURROUNDED, AT LEAST IN PART, THE CLEANING SPACE BETWEEN THE MEASURING SURFACE 2 AND THE MAGNETS 3. THE SURFACE EXTERNAL OF EACH MAGNET 3 IS ELECTRICALLY CONDUCTIVE, AT LEAST IN ITS PART 7 ARRANGED WITH REGARD TO THE MEASURING SURFACE 2. A GALVANOMETRIC DEVICE 9, 10, IS PROVIDED TO MEASURE THE CURRENTS PASSING IN THE MEASURING SURFACE 2, AND AT THE SURFACE OF THE AUXILIARY ELECTRODE 5 AND OF THE MAGNETS 3. APPLICATION TO IONIZATION DEVICES, IN PARTICULAR FOR AN ION IMPLANTATION TREATMENT, IN ORDER TO MEASURE THE IONIC FLOW WITH PRECISION, BY CONVENIENTLY ELIMINATING THE PARASITIC EFFECTS OF SECONDARY ELECTRONIC EMISSIONS.

Description

The present invention relates to a device for measuring a current

ionic arriving in the void on

a measuring surface.

  Modern technology offers many examples of devices equipped with measuring devices of the kind in question, and subjected to the effect of ionic currents free to propagate in a chamber where the vacuum prevails. However, it is important, for example for a surface treatment provided by ion bombardment, to know the value of the corresponding ionic current, and especially the ion density, that is to say the quotient of the ionic current by the value of the surface. affected, to ensure the effectiveness of the desired treatment. Often, it is also important to

  know the total number of ions that reach an over-

  face, a number called the "ionization dose", and that

  by integrating over time the value of the

  ionic flux measured on the surface in question.

  The ion implantation technique is a recent field of application for ion current measurements. Indeed, the physical properties of the surface

  of a body modified by ion implantation

  to a large extent to the ionization dose. The ion density ("A" per cm 2) can be measured by measuring the electrical current from a given ionbombarded surface probe. In the current state of the

  technical aspects, mainly ionic currents

  with intensities between 10 3 and 10 09 am-

  fathers. If the ion bombardment undergone by the surface of

  sure causes on it the formation of a plasma, or a

  secondary electronic broadcast, various special provisions

  are needed to accurately perform

  the desired measure. Indeed, this measure is made difficult.

  particularly because the electronic issuance

  that secondary from the substance of which the measurement surface is made also depends on the temperature of this surface and the residual gas pressure existing in the vacuum chamber. Measurement error due to emission

  secondary electronics comes mainly from the

  significant ionic current caused by this

  program. If, on the other hand, to measure by

  tation, an electronic circuit with high

  the measuring surface is positively charged by the positive ions which reach it, to the point of

  that the secondary electronic emission is in part

  removed, which again leads to a reduction in

  current ion current. To reduce the effect

  secondary electrons circulating in the vicinity of the sur-

  measuring face, there is already a known device which

  is to provide this surface with a Faraday cage. But such a solution causes difficulties, especially on industrial production facilities where it is not

  possible, for reasons of lack of space,

  put Faraday cages in the immediate vicinity of the

  measuring face. The simplest is then to provide a secondary electrode surrounding the measurement surface, and to attenuate the secondary emission in a wide range.

  measure, provided that a negative

of this electrode.

  It is also known that a magnetic field of about

  100 Gauss, transverse to the direction of the ion beam

  incident, sufficiently mitigates the secondary electronic emission, to the point that it is often no longer necessary to use a Faraday cage, or another kind of protective electrode. In this respect, we have seen the effectiveness of magnetic fields whose lines of force emerge from the measuring surface to enter another place, so that the electrons of the secondary emission remain captive under the "tunnel" formed by the lines of force formed vaulted above the surface of

  and thus find themselves obliged to return to the surface

this measurement.

  It should be noted that "measuring surface" here also refers to the surface of parts or substrates to be treated by ion bombardment, the surface on which it is intended to

measure the incident ion flux.

  But, as already indicated, the known devices mentioned above are not usable when not available, immediately next to the measurement surface.

  re, the space needed for electrodes and

  mants quite bulky. However, this is often the case when it comes to setting up afterwards a device to measure the

  ion current in an existing facility.

  In addition, measurement errors can be caused by electrons that do not come from the measurement surface, such as secondary electrons, but that come from traces of gas existing in the vacuum chamber, subjected to a partial ionization effect by the

  ion beam. When these "foreign" electrons reach

  on the measuring surface, they cause an attenuation

sensitive ion current.

  The object of the invention is to remedy the difficulties

  have just been exposed, in order to achieve a

  tif to measure an ion current arriving in

  the vacuum on the measurement surface, with better accuracy

  by removing measurement errors due to

  electronic noise, without having to have protective magnets close to the surface

measurement.

  The device according to the invention comprises, in

  of the measuring surface subject to an incident ionic beam, an auxiliary electrode and magnets

  to produce a transverse magnetic field at the direction

  tion of the aforementioned ion beam. This device is

  because it presents in combination

  following: (a) each magnet is spaced a distance away from the measurement surface, and the auxiliary electrode at least partially surrounds the space between

  measuring surface and magnets; (b) the external surface

  each magnet is electrically conductive, at least in its portion disposed opposite the measuring surface; (c) a galvanometric device is provided to measure the

  current passing through the measuring surface and over-

  face of the auxiliary electrode and magnets.

  The advantageous arrangement of the magnets, thus made possible by the invention, at a relatively large distance from the measurement surface, provides more space for mounting magnets.

  bigger and more powerful, with a better effect of

  tion. In addition, thanks to the invention, it is easier to

  make the support of the measuring surface mobile, in fact

  for example, this support by means of a rotating drum,

  on the inner surface of which the parts to be

  b. Another advantage of the device according to the invention is that one can use relatively powerful electromagnets, without their significant heating causes unacceptable heating of the parts to be treated, thanks to the relatively larger distance of these parts relative to magnets. In any case, the place we have then

  protects the parts by a cooling device

appropriate disposal.

  Other features and advantages of the invention

  will again emerge from the description of a mode of

  preferred embodiment, presented hereinafter by way of non-limiting example, with reference to the single appended figure which is

  a diagram of a device according to the invention for measuring

  ionic current arriving in the vacuum on a surface

this measurement.

  In the schematic embodiment of the

  figure, the measuring device is shown only

  by its essential parts serving to realize the me-.

  sure wanted. The device is located in an enclosure under

  * empty, for example part of an installation of im--

  ionic planting ,. and comprises a support 1 for holding a substrate having a surface 2 which constitutes a measurement surface with respect to an incident ion beam, the arrival direction of which is represented by an arrow. Two

  magnets 3, associated with the measurement surface 2 in the

  vacuum (not shown), are held in a fixed position by a fixing device (not shown), at a

  distance "a" opposite the plane of the measuring surface 2.

  Between the measuring surface 2 and the magnets 3 is mounted an auxiliary electrode 5, fixed for example to the magnets 3

  by connecting arms 4, and having a distance between

  ne "b", oriented transversely to the direction of the beam

ionic incident.

  In accordance with the invention, the values of the

  card "a" magnets 3 compared to the surface of

  2, and the internal distance "b" of the auxiliary electrode

  5, are chosen in such a way that the ratio is

  is greater than 1 and preferably greater than 1.

3 2

  For example, the auxiliary electrode 5 is made

  in two parts, namely a top plate and a

  bottom plate that surround together, at least in part

  space, the space between the magnets 3 and the measurement surface 2. Alternatively, the auxiliary electrode 5 may also be constituted by a substantially cylindrical envelope,

  almost completely surrounding the intermediate space

  cited, with the exception of a narrow gap similar to a

  slot, formed between the two parts of the electrode. The dis-

  internal flow "b" of the auxiliary electrode 5, transverse to the direction of the ion current arriving on the measuring plate 2, corresponds to the shortest distance between the two flat plates of the electrode, in its first mode

  of embodiment, or the internal diameter of the cylindrical envelope

  drique of the aforementioned variant of the auxiliary electrode.

  The auxiliary electrode 2 can be fixed separately

  to the walls of the vacuum chamber, either according to the case to the magnets 3, as shown schematically in the figure, either to the support 1, or even to the workpiece 2. In these two latter cases, the auxiliary electrode 5 is placed and removed with the support 1 or with the workpiece 2 and it is therefore necessary to provide a number of auxiliary electrodes each associated with a support 1 or a workpiece 2, to be brought to the following each other in the ionic current provided for the treatment

wanted from room 2.

  The device furthermore comprises

  Screen protector screen 8, to protect magnets 3

  and the auxiliary electrode 5 against the effects of the bombardment

  ionic effect, limiting the cross-section of the

  incidental dimension to the desired dimensions, to affect

  the exposed surface 2 of the workpiece.

  The magnets 3 which produce a magnetic field transverse to the direction of the incident ionic current consist for example of ferro-magnetic elements, or

  magnetic elements made of a suitable ceramic material

  asked. In each case, on the part of the surface of the

  mants 3 facing measurement surface 2, and subject to the effects of secondary ion emission from the

  measuring surface, an electrical coating

  driver 7; this conductive coating is normal-

  made without any particular arrangement by the surface of the body of each magnet 3, in the case where the device

  contains ferro-magnetic magnets.

  To measure the ionic current by means of the

  in accordance with the invention, the algebraic sum is calculated

  Brick of the various currents flowing on the measuring surface 2, the parts of the auxiliary electrode 5 and the surface 7 of the magnets 3, taking into account the sign which should be assigned each elementary current. For this

  These different currents can be measured by means of

  Separate conductors 9. The simplest solution is then to connect together the separate conductors 9, to make them lead to a single galvanometer 10, which allows

measure the resulting current.

  A particular advantage of the device according to the invention, as just described, is that the undesirable electrons, driven with the ions of the incident current towards the measuring surface 2,

  are kept away from this measuring surface under the ef-

-2462718

  fat of the transverse magnetic field of the magnets 3, arranged

  for this purpose at a sufficient distance from the surface

  this measurement. We choose for the distance mentioned above a

  they are sufficient to ensure the deviation of these undesirable electrons, which can thus reach neither the measuring surface 2, nor the auxiliary electrode 5, nor the magnets 3, so that they do not risk to distort the measurement that

we want to perform.

Claims (5)

REVENDI CATI ONS   1. Device for measuring ion current   arriving in a vacuum on a measurement surface, this   tif having, opposite the plane of the measuring surface   subjected to an incident ion beam, an auxiliary electrode   and magnets intended to produce a magnetic field   transverse to the direction of the ion beam   tooth, this device being characterized in that it has, in combination, the following particularities:   a) each magnet is spaced a distance apart by   the measuring surface and the auxiliary electrode   at least partially circumambulate the space provided between the measurement surface and the magnets b) the external surface of each magnet is electrically conductive, at least in its part arranged facing the measuring surface c) a galvanometric device is provided for measure   current passing through the measuring surface and over-   face of the auxiliary electrode and magnets.   2. Device according to claim 1,   characterized in that it has a ratio a greater than 1 b 3 "a" being the spacing of the median plane of the magnetic field from the measuring surface, and "b" being the shortest internal distance from the electrode auxiliary, in a   transverse direction to the incident ion beam.   3. Device according to claim 2,   characterized in that the ratio a is greater than 1 b 2   4. Device according to claim 1,   characterized in that the auxiliary electrode is integral with the magnets.   5. Device according to claim 1, characterized   in that the auxiliary electrode is secured to a   support for supporting the measuring surface.