Device for analysing a surface layer by means of ion scattering

Abstract

A device for analysing a surface layer by means of ion scattering. The device comprises an energy analyzer having two coaxial cylindrical electrodes. The axis of the primary monoenergetic ion beam coincides with the axis of the cylindrical electrodes. The back-scattered ions the path of which lie at a conical surface having half an apex of 180.degree. reduced by the back scattering angle are selected and detected for energy. The detector is annular. The primary ion beam is injected via the aperture in the center of the annular detector.

Description

The invention relates to a device for analysing a surface layer by means of ion scattering and comprising: means to produce a primary substantially monoenergetic ion beam, a diaphragm aperture for passing ions which are scattered at the surface layer at a previously determined angle with respect to the axis of the primary ion beam, and an electrostatic analyser and a detector to determine the kinetic energy of the scattered ions passed through the diaphragm.

Such a device is known from the U.S. Pat. Specification No. 3,480,774. In such an ion scattering spectrometer the surface layer to be examined is bombarded with a primary ion beam. The ions of said beam collide with the atoms of the surface layer, which collisions may in certain conditions be considered as being elastic. This means that the kinetic energy of an ion after the collision may be calculated by means of the laws of conservation of energy and momentum. If

E.sub.1 = kinetic energy of an ion prior to collision

E.sub.2 = kinetic energy of an ion after collision

M.sub.1 = MASS OF THE ION

M.sub.2 = MASS OF THE ATOM IN THE SURFACE LAYER AGAINST WHICH THE ION COLLIDES

.gamma. = M.sub.2 /M.sub. 1

.theta. = the angle of scattering, that is the angle between the velocity vectors of the ion prior to and after collision, then it holds if .gamma. > 1 as is known that

E.sub.2 = [ cos .theta. + (.gamma..sup.2 - sin.sup.2 .theta.).sup.1/2 /(1 + .gamma.)].sup.2 E.sub.1 From this it follows that m.sub.2 can be determined by measuring E.sub.2 if m.sub.1, E.sub.1 and .theta. are known and if it may be assumed that only single collisions take place. In an ion scattering spectrometer this is done as follows. A beam of ions, usually rare gas ions, of known mass m.sub.1 and known energy E.sub.1 is directed on the surface layer to be examine. A diaphragm is arranged so that the direction of scattered ions which pass through the gap enclose a known angle .theta. with the direction of the primary beam. The energy of the passed ions is measured in an energy analyser. At a given voltage at the electrodes of the energy analyser only scattered ions of a given energy E.sub.2 can pass the analyser. Given m.sub.1, E.sub.1 and .theta., said energy is therewith characteristic of the mass m.sub.2 of atoms in the surface layer which are hit by the primary beam. By varying the voltage at the electrodes of the analyser, a spectrum can be obtained of the types of atoms occurring in the surface layer. With given voltages on the analyser a peak in the signal is obtained which the detector supplies. The values of the peak is a measure of the relative quantity of the relevant atoms and the voltage on the analyser associated with the peak is a measure of the mass of the relevant atoms.

It is obvious that the angle .theta. must be accurately determined and the aperture in the diaphragm should therefore be so small that only few scattered ions are passed. In practice, .theta. should be determined to an accuracy of 1.degree. to 2.degree. by the diaphragm, which has for its result for the known device that only scattered ions within a space angle of 2.degree. .times. 2.degree. can be accepted so that only a very small signal is formed.

From the article "Zur Energieverteilung der von Protonen in Gasen ausgelosten Sekundarelektronen" in "Zeitschrift fur Physik" volume 147, pp. 228-240, 1957, an energy analyser is known having two coaxial cylindrical electrodes. Such an analyser has the advantage that the paths of the scattered ions which enclose a given angle with the axis of the analyser, with which axis the axis of the primary beam coincides, lie on a conical surface. By using an annular diaphragm having a gap which is 2.degree. wide and has a circumference of 360.degree., 180 times as many ions are accepted by the analyser as by the analyser which is used in the device described in the U.S. Pat. Specification No. 3,480,774. However, such an analyser could not be used so far in an ion scattering spectrometer because the primary ion beam must extend along the axis of the analyser and hence either the surface layer to be examined or the detector forms an obstruction for the primary ion beam.

It is the object of the invention to provide an ion scattering spectrometer in which an energy analyser can be used having two coaxial cylindrical electrodes. Another object of the invention is to provide an ion scattering spectrometer which gives a considerably improved mass separation between the atoms present in the surface layer and produces a considerably larger signal.

According to the invention, a device of the type mentioned in the first paragraph is characterized in that the electrostatic analyser comprises two substantially cylindrical hollow coaxial electrodes, that the axis of the primary ion beam coincides substantially with the axis of the analyser, that the diaphragm aperture is substantially annular and is coaxial with the analyser and has a position to pass ions which are scattered over an angle exeeding 90.degree., and that the detector is substantially annular and coaxial with the analyser and surrounds the primary ion beam.

The ions are preferably detected at an average angle of scattering between 137.degree. and 150.degree.. With said angles of scattering it is possible to make a second order focus of the scattered ions on the detector without the scattered ions first passing the axis of the analyser. A second order focus has the advantage that ions from a comparatively wide region around the average angle of scattering are focused on the detector.

The invention will be described in greater detail with reference to the accompanying drawing, of which

FIG. 1 is a perspective view, partially broken away, of a cylindrical analyser for a device according to the invention, and

FIG. 2 is a diagrammatic sectional view of a device according to the invention.

In FIG. 1, a primary, substantially monoenergetic ion beam 1 impinges upon a target 2 with an energy of, for example, a few hundred eV. The ions should be selected for mass, charge and energy, which may be carried out with means which are known from the prior art and which need no further explanation. Rare gas ions, for example helium or neon ions, are preferably used. An advantage of said ions is their large ionisation energy; this results in a fair chance of their charge being neutralised during the collision, which, it is true, results in a small number of scattering ions but also reduces the possibility of detection of multiple collisions which spoil the measurement. A device according to the invention has just the property that also in the case of a small number of scattered ions sufficient signal is still produced by the detector.

The axis of the primary ion beam 1 coincides with the axis of an energy analyser 3. The energy analyser 3 comprises two coaxial cylindrical electrodes 4 and 5. The ions of the beam 1 collide against atoms in the surface layer of the target 2 and are scattered. They lose a certain quantity of energy which depends upon the angle of scattering and the mass of the atom in the surface layer. The energy analyser 3 measures said loss of energy for a given angle .theta. (FIG. 2) which exceeds 90.degree.. So here we have to do with back scattering. The angle .theta. may be, for example, 141.degree. so that the ions accepted by the energy analyser describe paths over the surface of a cone with an apex of 78.degree.. The angle .theta. is determined by the position of the diaphragm aperture 6 in the cylindrical electrode 4 relative to the target 2. In the radial electric field between the electrodes 4 and 5 the scattered ions describe quasi-parabolic paths, a few of which are denoted by 7, 8, 9 and 10, and can pass through the second diaphragm aperture 11 only at a given potential difference between the elctrodes 4 and 5 which is a measure of their energy. The energy analyser 3 focuses ion paths which start in the point 12 on the target 2 in an annular focus behind the diaphragm aperture 11. At this area an annular detector 13 is arranged to detect the scattered ions. Said annular detector 13 surrounds the primary ion beam 1 so that same can reach the target 2 without hindrance.

FIG. 2 shows for further explanation a sectional view of a device according to the invention namely along a plane through the axis of the primary ion beam 1. The beam 1 is produced by an ion source 14 extracted by an extraction electrode 15, focused by focusing electrodes 16 and 17 and selected for mass by a mass filter 18. The energy analyser 3 has the same reference numerals as in FIG. 1.

For a good operation of the energy analyser, the electric field between the electrodes 6 and 7 should be everywhere equal to the field between two infinitely long coaxial cylinders. Since the cylinders in practice have a restricted length, electrodes should be provided so as to establish the boundary conditions for the field and possibly slightly correct the field. For that purpose the cylinders 4 and 5 are closed by means of the plates 19 and 20 which are not shown in FIG. 1 for clarity. The plates 19 and 20 introduce an average potential between that of the cylinders 4 and 5. It is also possible to divide the plates 19 and 20 into several electrodes having different potentials so as to obtain a better approximation of the required electric field. The plates may also be manufactured from a material having a large electric resistance and be connected to the cylinders 4 and 5 so as to obtain a uniformly varying potential.

As already noted, a device according to the invention operates with back scattering. Although fewer ions are scattered in the backward direction than in the forward direction, the device according to the invention is just very suitable to detect small quantities of ions. The described back scattering on the contrary has important advantages. First, when the primary beam impinges upon the target approximately perpendicularly, the possibility of sputtering is so much smaller so that the surface layer of the sample is less damaged by the primary beam. At an angle of incidence of 90.degree., sputtering often occurs only at 60 eV, while at an angle of incidence of 45.degree. 10 eV is already sufficient. Second, the possibility of multiple collisions which spoil the measured result is much smaller with an approximately perpendicular incidence of the primary beam on the target than with angles of incidence smaller than 90.degree..

In the derivation of the collision formula used, the movement of the atoms of the target has been neglected. This movement gives a widening of the peak in the signal of the detector. Cooling of the target may thus be of advantage so as to be able to distinguish from each other peaks of the spectrum which are situated immediately beside each other.

A low-energy electron gun or a filament may be arranged near the target so as to ensure in known manner for space charge compensation. It is furthermore possible to slowly peel the target layer by layer by means of a separate ion beam or by means of the primary beam with increased intensity, so as to be able to thus analyse deeper situated layers also.

The inner diameter of the electrode 5 in the device shown in the drawing is 125 mm and the outer diameter of the electrode 4 is 50 mm. The distance between the point 12 on the target 2 and the centre of the annular detector 13 is 90 mm. The angle .theta. is 141.degree.. The electrode 4 is earthed in connection with the transport of the primary ion beam 1. For selecting ions with an energy of V electron volt the potential of the electrode 5 (relative to the electrode 4) should then be V volt.

The ion source 14 supplies a flow of ions of the order of magnitude of a few nanoamperes to a few microamperes with an energy which is adjustable from a few tens of volts to a few kilovolts. As already noted, rather high requirements are imposed upon the energy spreading of the ions which emerge from the ion source. In practice this should be between 0.1 and 1.0 eV, which is possible with ion sources known from the prior art.

The mass filter 18 preferably consists of the known Wien filter. In this filter the beam is subjected to the influence of an electric and a magnetic field the directions of which extend perpendicular to each other and to the axis of the beam. This has for its result that of the substantially mono-energetic ion beam only ions having one given mass are not deflected and can pass the filter. The other ions are deflected indeed and are intercepted at some distance from the mass filter by means of a diaphragm.

The detector 13 must detect such a small ion flow that direct current measurement is not possible with sufficient accuracy. In the embodiment a so-called channel plate is used as a detector which consists of a plate having a very large number of extremely thin secondary emitting channels. Per incident ion, which after passing the diaphragm aperture 11 is post-accelerated to an energy of a few kilovolts by giving the detector a suitable potential relative to the electrode 4, such a channel plate supplies 10.sup.4 to 10.sup.8 electrons at the output. Per ion said electrons form an electron pulse. The pulse frequency is measured with known electric means and thus forms a measure of the number of ions entering the detector per second and hence of the ion flow.

Claims

What is claimed is:

1. A device for analyzing a surface layer by means of ion scattering, comprising:

a. means for producing a substantially mono-energetic primary ion beam and comprising an ion source;

b. a substantially annular diaphragm aperture adapted to pass said ions scattered at said surface layer at a predetermined angle with respect to the axis of said ion beam, said angle exceeding 90.degree. with respect to the original direction of said primary ion beam;

c. an electrostatic analyzer comprising two substantially cylindrical hollow electrodes coaxially disposed with respect to each other, said analyzer having an axis substantially coincident with said primary ion beam axis and being substantially coaxial with said diaphragm aperture, said ion source being disposed at a first end of said analyzer and said surface layer being disposed at the opposite second end of said analyzer; and

d. detector means for determining the kinetic energy of those of said scattered ions passing through said diaphragm aperture, said detector means comprising a substantially annular detector element that is coaxial with said analyzer and extends around said primary ion beam axis, said detector being removed from said surface layer in the direction of said ion source.

2. A device as in claim 1, wherein the average value of said angle is about between 137.degree. and 150.degree..