BACKGROUND OF THE INVENTION

A number of works have covered the field of devices aimed at obtaining a spectrum of charged particles as a function of their energy, e.g. "Principles of Electron Optics" by P.H. Hawkes and E. Kaspar, Academic Press, New York, 1989. In addition D. Roy and D. Tremblay, Rep. Prog. Phys. 53, 1621 (1990) is a useful reference.

A common use for charged particle analysers is in the field of surface analysis on a scale of up to few nanometers, such as Auger electron spectroscopy and X-ray Photoelectron spectroscopy. The usual method for acquiring a spectrum in either technique is to step through the spectrum one point at a time using a single detector. A faster approach is to use a series of detectors which will provide a higher signal to noise ratio for the same collection time, or allow a faster rate of scanning the spectrum. However, the range of energies that can be acquired is typically quite small with the energy range that can be acquired simultaneously being about 0.1 or less of the entire energy range of the spectrum that is normally required. For recent review of current methods in acquiring such spectra see M. Seah in Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy, Ed D. Briggs and J T Grant. IM Publications, 2003, chapter 7, ppl67-191and chapter 13, pp 345-376. An alternative approach was effected with the invention of the Hyperbolic Field Analyser (HFA) M. Jacka, M. Kirk, M. El Gomati and M. Prutton Rev. Sci. Instrum., 70, 2282, (1999). This device has the advantage of being able to collect a whole spectrum simultaneously. F. Read extended this approach with a similar device but with cylindrical symmetry (F. Read, Patent No. WO 00/77504 Al).

However, for these devices, the landing position of the electrons on the detector has a high dependence on the angle of take off of the electrons. This means that the aperture of the analyser has to be kept narrow which limits the sensitivity of the device.

Another approach to the determination of the energy of an electron is to use a 180° magnetic electron spectrometer (see K.D. Sevier "Low Energy Electron

Spectrometry", (Wiley-Interscience) (1972) for a review). This approach was first carried out by Rutherford and Robinson Phil. Mag. 26, 171 (1913). The principle of the 180° spectrometer has also been used to carry out mass spectrometry. (Dempster, Phys. Rev. 11, 316 (1918)). Later, Siegbahn (see K. Siegbahn "Electron

Spectroscopy for atoms, molecules and condensed matter" Nobel lecture, 8

December, 1981,

http://nobelprize.org/nobel prizes/physics/laureates/1981/siegbahn-lecture.pdf) used a double focusing magnetic spectrometer to acquire X-ray Photoelectron Spectra. However, the instrument acquired electron spectra in a serial manner by gradually ramping the magnetic field strength such that a single detector could be used to collect the electrons at a series of different electron energies. Despite the inherent advantages of using the 180° spectrometer for parallel acquisition, this device has never been applied to the acquisition of electron spectra in parallel for techniques such as Auger electron spectroscopy and X-ray Photoelectron Spectroscopy.

SUMMARY OF THE INVENTION

A preferred device discussed here is a charged particle energy analyser using a largely homogeneous magnetic field to disperse the charged particles and cause them to land on a detector such that the landing position on the detector is dependent upon the energy of the charged particle. In the case of the charged particle being an electron, the source of electrons that pass through the analyser are generated by an electron column (usually in a scanning electron microscope). The direction of the magnetic field of the analyser is arranged to be parallel to the axis of the electron column to reduce the deflection of the primary electron beam and to ensure symmetry of the electron column. Considering the more general case for all charged particles when analysed using a preferred embodiment of the present invention, these charged particles would pass through an aperture and move in a helical manner towards the detector (see Figure 1). Said charged particles would arrive at different position depending on their energies and angles of emission. This configuration means that charged particles can be acquired over a large range of energies and angles simultaneously.

The charged particle detector used in preferred embodiments of this invention is intended to be flat, but could be curved in order to take into account the possibility that the magnetic field may not be homogeneous due to the influence of the magnetic field of the objective lens or other necessary devices that could lead to inhomogeneities of the instrument magnetic field.

The detector is intended to be mounted such that the surface normal of the detector is perpendicular to the direction of the magnetic field. However, the detector could also be canted in any direction to take into account magnetic field inhomogeneities or other disturbing factors.

The magnetic field could be generated using coils or wires carrying electric currents and/or permanent magnets. Soft iron could be used to direct the magnetic fields from the coils or permanent magnets to ensure homogeneity of the magnetic field.

In one configuration (embodiment) of this invention, as depicted in Figure 1 (in the case of electron bombardment and electrons passing through the analyser), the electrons pass through a narrow slit (e.g. in a foil or plate) before striking the detector. The electrons execute a 180° rotation before striking the detector, making it similar to other well known devices in this respect. However, better energy resolution can be obtained if the detected electrons execute an angle greater than 180° for electrons which have a component of velocity parallel to the magnetic field. Hence a slit which is curved to take into account this effect results in better energy resolution (Figure 4). When the take off angle, φ, is close to 36.5° and the initial launch angle, a, is close to 129°, the landing position of the charged particles on the detector follows along the same line when the angles φ and a are changed. This can be considered as second order focusing and the aperture can be made much wider to take into account these conditions. This is depicted in Figure 4. The trajectory of charged particles for different angles of φ for optimum energy resolution is depicted in Figure 7.

The second order focusing effect could be utilised in a variety of other charged particle energy or mass analysers by injecting charged particles into a magnetic field from a largely parallel source of charged particles. One such possible manifestation is depicted in Figure 11.

Although preferred embodiments of the invention are primarily intended for use with a scanning electron microscope, the electrons that are energy analysed could be excited by a number of means such as X-rays, ions or other charged particle or neutral particle beam.

Characteristic features of certain preferred embodiments of the invention are set out in the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 is a side view of a first embodiment of a charged particle analyser according to one example of the invention. Figure 2 shows a plan view of an embodiment of the invention with the aperture and detector in the same plane. The Figure shows three different charged particle trajectories for a single charged particle energy. Figure 3 shows a diagram giving the meaning of the angle symbols in the equations used in this document, a is the take off (injection) angle with respect to the x axis and φ is the take off (injection) angle with respect to the z axis .

Figure 4 is similar to Figure 1, but shows a possible modification to the slit, causing rotation angles greater than 180° for reduced take off angles φ. The width of the slit is much greater for φ in the region of 35°.

Figure 5 shows the locus of landing positions on the detector for equal energies but varying take off angle φ and the loci of three different landing positions on the detector as the angle a is moved such that the electrons are swept across the slit. The three different loci correspond to three different starting azimuthal angles a. Note that for α π/2, this locus is more closely aligned with the elliptical locus of equal energy but varying <p. Figure 6 shows the landing position of electrons with varying meridional angle φ for angles of rotation of 180° (i.e. a = 90°) forming an ellipse. In addition, the landing positions of electrons for fixed φ but varying energy are also shown forming nearly straight lines starting at the origin. Figure 7 shows the trajectory of electrons for optimum energy resolution. The angles of rotation gradually increase as φ reduces.

Figure 8 shows the landing position of electrons for optimum energy resolution for a fixed energy but varying meridional angle φ. The angles of rotation (and therefore azimuthal take off angle a) gradually increase as φ reduces in accordance with equation (7). Figure 9 shows an elastic peak as acquired on the sensor, where the brightness reflects the electron intensity while position reflects the electron energy.

Figure 10 shows a secondary electron spectrum of a solid displaying the peak of elastically reflected electrons at 900 eV, where said secondary electron spectrum is determined from Figure 9.

Figure 11 depicts a possible arrangement for a charged particle analyser taking advantage of the second order focussing mode. The charged particles are injected at approximately 35.5° with respect to the magnetic field and execute helical motion and turn through approximately 258° before striking a two dimensional charged particle detector.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, the polarities of the applied magnetic fields are chosen for the analysis of negatively-charged particles, and in the embodiments of Figures 1 to 7 the charged particles are assumed to be electrons. It will, of course, be appreciated that positively charged particles may be analysed by reversing the direction of the applied magnetic field or moving the detector to an appropriate position.

It is well known that an electron moving in a magnetic field of constant strength and direction performs circular motion. It can easily be shown that electrons moving perpendicularly to the direction of the magnetic field pass through a first order focus (the point at which they reach maximum distance from the source) after describing a semicircle, which is the basis of 180° magnetic spectrometers. The circular motion is independent of energy, but the radius of the circle is dependent on the square root of the particle energy. Particles which land on a detector such that they have undergone a rotation of 180° (for particles moving perpendicularly to the magnetic field direction) will be dispersed according to their energy. For particles moving at other angles with respect to the magnetic field direction, the optimum energy resolution will be obtained at different angles of revolution to 180°. Hence the aperture is, ideally, not a vertical slit, but one which is curved in order that the optimum energy resolution is maintained at all take off angles.

If charged particles are arranged to move in a magnetic field in a direction perpendicular to the magnetic field with parameters as given in Figure 3, then it can be shown that the variation in the landing position, x, as the azimuthal angle, a, is varied is given by:

Λχ

— = l - cosAcr (1)

JC

The formula for the radius, R of a charged particle travelling with energy E, in magnetic field of strength B can be easily calculated as

R =^l (2)

Be

The energy resolution of the analyser can then be derived as a function of the spatial resolution

E x

A table showing the ratio, Δχ/χ, for various ranges of azimuthal angle, Δα, is shown in table 1. The energy resolution, ΔΕ/Ε, as a result of the range of landing positions is also shown in table 1.

Δα Δχ/χ ΔΕ/Ε Δχ/χ ΔΕ/Ε

(degrees) MA MA HFA HFA

1 .00015 0.0003 .0012 0.0024

2 .00061 0.0012 .0047 0.0094

3 .0013 0.0026 .0104 0.0208

4 .0024 0.0048 .01835 0.0367 Table 1. The variation of landing position as the angle a is changed and the resulting energy resolution for an embodiment of the present invention (MA) and the above- mentioned hyperbolic field analyser (HFA). The calculations here are for electrons travelling perpendicularly w.r.t. the magnetic field direction.

If x is the horizontal direction, and z is the vertical direction (see Figure 3), E is the energy of the charged particle, m is the mass of the charged particle, B is the strength of the magnetic field and e is the charge on the charged particle, then it can be shown that the locus of points striking the detector that have the same energy but different take off angle, φ, and striking the detector after a 180° rotation describes an ellipse given by the following equation (see Figures 2 and 6).

The landing positions of charged particles of different energies, but the same take off angle, φ, are given by the f llowing equations (see Figure 6).

Equations (5) and (6) are valid only for charged particles undergoing a 180° rotation.

More general equations for any value of a and <p, but not including the finite length between source and aperture (d) are given by:

sin ² a a ² (Be) ²

l^lmE

-sin e ⁾ sin or (8)

Be

iJlmE

a cos ^ (9)

Be The limitation in the range of me azimuthal angle, a, is provided by the aperture.

For operation in a scanning electron microscope, the magnetic field is so arranged to be in the same direction as the axis of the electron gun so as to reduce the effects of the magnetic field upon the proper operation of the electron gun. Typically, the strength of the applied magnetic field would be 80-100 Gauss.

As shown in equation (4) charged particles of equal energy but varying take off angle will land on the detector and describe an ellipse when executing a 180° rotation. This implies that the electrons passing through the aperture will have the best resolution when the locus of charged particles over the angle Δα that pass through the aperture coincides as closely as possible to the ellipse given in equation (4). When the electron motion is out of the plane perpendicular to the magnetic field, the angle through which the electrons must travel for optimum energy resolution is no longer 180°. For optimum energy resolution, it can be shown that the azimuthal angle, a, (for d = 0) can be given by:

tana ₇ ,

= -tan ² <0 (10)

a

Where φ is the meridional angle as shown in Figure 3. Hence the "vertical slit" should not be vertical at all, but curved so as to optimise the energy resolution for as large a range of meridional angles, <p, as is reasonably possible (see Figure 4).

It can also be shown that the second differential (d ² z/dx ² )of the landing locus for varying a can be made to be the same as second differential of the landing locus for varying φ at one angle. This effectively corresponds to second order focusing. The second order focusing angle is satisfied by the following equation (for d = 0).

. 3 . tan ² a

tan = tana (11)

It can be shown that α ~ 129° satisfies this equation. The corresponding value of φ as given by equation (10) is ~36.5°. Thus injecting electrons into a homogeneous magnetic field at these angles can be accomplished with an especially large aperture and electrons with all energies will benefit from the second order focussing. In this specification, the verb "comprise" has its normal dictionary meaning, to denote non-exclusive inclusion. That is, use of the word "comprise" (or any of its derivatives) to include one feature or more, does not exclude the possibility of also including further features.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.