AU539588B2 - Angular resolved spectrometer - Google Patents

Description

ANGULAR RESOLVED SPECTROMETER

BACKGROUND OF THE INVENTION

This invention provides a charged particle energy analyser of the electrostatic type having the capability of accepting charged particles emitted by a source over a wide range of angles in such a manner that the angle of emission of an individual charged particle may be determined from its position of arrival at a position-sensitive detector.

In may forms of spectroscopy involving the detection of charged particles, such as electrons or ions, which have been ejected from some source, such as gases or solids, it is necessary to determine the energy distribution of the charged particles. Numerous energy analysers have been des¬ cribed in the literature which are capable of determining the number of charged particles accepted into the analyser as a 'function of the kinetic energy of the particles, vide: K.D. Sevier, "Low Energy Electron Spectrometry", published by Wiley, New York, 1972. Such analysers may be categorized in two ways for the purposes of describing the instrument of the present invention: (a) by their use of electrostatic or magnetic fields as the means whereby charged particles are accepted or rejected on the basis of their energies, and (b) by the angular acceptance capability of each analyser. As an example of a spectroscopy using electrostatic analysers, photoelectron spectroscopy will be used-

Solid state photoelectron spectroscopy involves the energy analysis of electrons emitted from solids when mono¬ chromatic photons impinge on them. The usual photon energies used are the Alkα X-ray line of 1486.6 eV or the noble gas discharge lines of He at 21.22 eV or 40.81 eV. More recently continuum synchrotron radiation sources have been used in conjunction with monochromators so that photons of any chosen energy may be employed. The most usual form of analyser presently used is a parallel plate capacitor shaped in such a way that only electrons of a single energy arrive at the detector. The two most preferred designs are concentric hemi- spherical plates or concentric cylinders. These are said t be double focusing which means that electrons of the same energy will arrive at the focus point even if they diverge from the main path in either of two perpendicular planes.

^• Photoelectrons are emitted from solid surfaces whe σ o illuminated with light, for example, UV 304A or 584A. The electrons have energies and momenta which .can be related to their initial states in the solids. The angles at which el trons are emitted from the surface of single crystal sample depend upon the initial state of the electron within the so By measuring the angular distribution (energy and angle of emission) of the photoelectrons, the full energy-momentum states (band structure) of the material can be determined. This is currently providing the most direct experimental li with theoretical calculations of electron states in solids, and provides experimental confirmation or criticism of the extensive theoretical literature.

Angular resolved spectrometers are currently comme cially available. The analyser used has an acceptance cone limited by slits to the required angular resolution, approx imately -2 , and is usually mounted on a rotatable plate so that electrons leaving the surface at different angles can measured successively. A single crystal sample is mounted- a known orientation in the spectrometer, and the analyser s at known angles to the crystal axis and rotated around the specimen to determine the energy spectrum at each setting.

A spectrum of counts against energy taken from +-90 from the crystal surface normal in steps of 2.5 for a maxi of 73 different positions, typically requires about 30 minu at each position for He 21.22 eV photons. Also, there are attendant problems of surface cleanliness as the surface of the crystal adsorbs gas atoms from the vacuum, which progres ively degrades the spectrum in a few hours. Further, becaus of variations of light intensity, it is difficult to relate precisely the intensity of individual spectra. SUMMARY OF THE INVENTION

The present invention provides an angular resolved spectrometer which is capable of analysing charged particle energy at a substantial number of angles simultaneously, 5 without the necessity of rotating the analyser, that is, capable of simultaneously obtaining spectra with a resolution of -1.0 for a range of angles of emission of the order of 340 . This minimizes the analysis time and thereby avoids the problem of maintaining surface cleanliness over a long 10 period, besides enabling a direct comparison of individual spectra.

An angular_.resolved spectrometer in accordance with the present invention is characterized by: (I) concentric toroidal sectors which move charged particles with 'emission

15 angles - < <+ , any β value,, and a chosen energy, entering at a path midway of the inlet end of an open-ended annular toroidal-contoured passageway formed by said concentric toroidal electrode sectors and between which an electrical field is arranged to be established, so that charged parti-

^•20 cles with said energy and emission angles ( ,3) will be refocused such that those charged particles with differing α angles are strongly refocused but those charged particles with differing β angles are only weakly refocused, thereby to retain the required β angular information at the α focus

25 plane and provide a focus of charged particles into ring form; and (II) a. charged particles position-sensitive detector which registers the focus of charged particles in ring form and generates signal pulses according to the position of arrival of the charged particles on the detector.

30 More particularly, the present invention provides an angular resolved spectrometer having the capability in¬ dicated, which comprises in axial alignment: (A) a charged particles input focusing section embodying a slitted elec¬ trode which defines an angle α and refocuses all charged

35 particles from the analysis source with emission angles -α < <+ and a chosen energy; (B) an energy resolving electrode section embodying concentric toroidal electrode sectors which move said refocused charged particles with a chosen energy, entering at a path midway of the inlet end o an open-ended annular toroidal-contoured passage-way formed by said concentric toroidal sectors and between which an electrical field is arranged to be established, so that charged particles with said energy and emission angles (α,β will be refocused such that those charged particles with differing α angles are strongly refocused but those charged particles with differing β angles are only weakly refocused thereby to retain the required angular β information at the α focus plane and provide a primary focus of charged partic into ring form; (C) a charged particles output focusing sec tion embodying a slitted electrode which defines the focal plane of the charged particles emitted from the outlet end of said annular toroidal-contoured passageway and provides a secondary focus of charged particles into ring form; and (D) a charged particles registering section embodying a charged particles position-sensitive detector which registe the focus of charged particles in ring form and generates signal puls.es according to the position of arrival of the charged particles on the detector.

In further describing the spectrometer of the pres invention, it is convenient to identify two planes characte ized by angle's α,β as indicated in Figs..l, 3 and 5 of the accompanying drawings. By virtue of the toroidal geometry of the energy resolving electrode section, charged particle can be accepted into tire sp-ectrometer for analysing sensibl all angles β<360 , however, only those charged particles emitted from the analysis source into a cone of half angle about the horizontal plane will be accepted (α ~ 2°) .

A feature of the spectrometer of the invention is that charged particles of chosen energy are refocused onto the charged particles position-sensitive detector for those particles originally within the acceptance cone defined by α , but there is sensibly no focusing in terms of the angle Stated in another way, there is a one-to-one correspondence between the emission of charged particles at a particular angle β, and a range of angles -α <α<+α , and the arrival of that fraction of such particles as was emitted with a selected value of kinetic energy, at a unique point on the detector. For an analysis source which emits charged particles for all angles o < β < 360°, such particles as have the correct, emission energy will be refocused as an annular (circular) pattern on the detector.

Means for measurement of differences in arrival times of the signal pulses is preferably employed to determine the angle β at which the charged particles were emitted from said analysis source. Signal pulses generated by the charged particles refocused as an annular pattern on the detector can be electronically processed in any suitable manner to provide data as a function of energy at a particular angle. Thus, the signal pulses can be processed into digitized time differences and loaded into the histogram memory of a control computer so that it contains counts as a function of angle for one particular energy, then reorganized in the data memory to give counts as a function of energy at a particular angle, with repeats until satisfactory statistics have been obtained.

_»PREFERRED EMBODIMENT OF THE INVENTION

In accordance with a preferred embodiment of the present invention, the spectrometer comprises five major sec¬ tions as set out below:

1_: A set of slitted electrodes of cylindrical symmetry which serve to define the angle α and to refocus all charged parti- cles from the analysis source with emission angles -α <α<+

^J ^ o o and a chosen energy, at a path midway of the inlet end of the annular toroidal-contoured passageway defined by the surfaces of the concentric toroidal sectors. By varying the voltages applied to this input lens of electrodes, charged particles of various energy can be brought to a focus at the entrance - to the toroidal-contoured passageway. This input lens has been designed using the data disclosed by E. Harting and F. Read, "Electrostatic Lenses", published by Elsevier, Amsterd .1976, which is applicable for planar aperture lenses, as th first approximation for the design of the present cylindrica elements, the design being finalized using numerical analysi based on a relaxation procedure, vide: T. Mulvey and M.J. Wallington, Reports on Progress in Physics, 3_6_, 347-431, 197

2: An energy resolving section consisting of two concentric sectors of toroids spaced-apart so as to form a toroidally- contoured passageway and between which is established an electrical field. Charged particles with an energy E enter this field at the mid point of the passageway and perpendicu to it will move in an almost circular path of radius a equi distant from each toroidal surface if the electrical potenti on each toroidal sector with radii r, , r„ are:

2E

^V(rl,2⁾ = (2a + πR) In ^ao ^(2rl,2 ÷ ^πR)

TΓR ^rl,2 ^(2ao ^{+ πR)}

where V(r, ~) is the voltage on an electrode of radius r, or E is the required pass energy of the analyser in electron v a is the radius of the main path, R is the radius of rotati of the generating circle of the toroid, and r, and r~ are th radii of the generating circles of the toroidal electrodes. Charged particles with the above energy E which deviate in angle (α) from the perpendicular entry path and for any angl where α is the angle of deviation in a plane containing the of the spectrometer and β is an angle in a plane perpendicul to this axis, will be refocused by the toroidal energy resol section. An intermediate object having been established by the input lens system near the entrance to the toroidal sect this section then strongly refocuses those charged particles with differing α angles but only weakly refocuses charged particles with differing β angles, thereby retaining the re¬ quired β angular information at the α focus plane.

cc; 3: A.set of slitted electrodes of frusto-conical symmetry and consisting of: (i) a second focal plane electrode which serves to define the output slit size, and (ii) a two element accelerating lens system for the charged particles.

The α focal- points of the toroidal section lie on a circle defined by a slit in said focal plane electrode. The position of the focus is calculated to a first approximation using ollnick's general theory of analysers, vide: H. Wollnick, "Focusing of Charged Particles", ed. A. Septier, Vol. II, published by Academic Press, N.Y., 1967. This depends on the toroid sector angle θ, the radii of the toroidal sections, and the generating radius of the toroids R. The energy resolving power depends on all radii and on the sizes of the input and the output slits of the analyser.

The two-element accelerating lens system, shaped as frusto- conical sections, functions to accelerate the charged particles to a suitable energy (300-500 V) for transfer of the ring-form focus of charged particles to the position-sensitive detector. •This lens system is designed using the normal criteria for slit ienses (Harting and Read, supra) as a first approximation and incorporates adjustments allowing for the actual lens geometry being conical.

4_: A microchannel amplifier plate (Galileo model 3040-B) whic under electrical potential amplifies the charge delivered by each incident charged particle by a factor of -^10 and ejects the charge for registering on the charged particles position- sensitive detector.

5_: A charged particles position-sensitive detector which is arranged to be at a higher electrical potential than the exit potential of the microchannel amplifier plate and is disposed below the microchannel amplifier plate to receive the amplified pulses ejected onto the detector.

The detector follows the usual technology for position-sensitiv detectors but is of novel geometry, that is, it is different from other con gurat ons n t at as t e na ana yser fo is a ring, the detector is in strip-form and in the shape a section of an annulus from whose ends the signal pulses derived.

The detector preferably consists of a plate containing a plurality of separate annular resistive strips, say, four, though only one of these is used at any time. The remaini strips may be brought into use by adjusting the vertical position of the microchannel amplifier plate and detector plate in the event of damage occurring to a particular par of the microchannel amplifier plate.

In the preferred practical form, the detector consists of thin ceramic plate (0.6 mm thick) coated on the top side with one or more resistive coatings to which sensing elec- trodes are attached and on the bottom side with a conducti layer which is earthed.

The detector plate acts as a distributed RC delay line and when a charge pulse strikes the detector strip at a given point, a charge flows to both ends of the detector strip. The arrival time of each pulse at the ends of the detector strip depends on the distance travelled so that by measurin the difference in arrival times, the position of arrival of the charge on the annular strip can be determined, vide: E. Mathieson, K.D. Evans, . Parkes and P.F. Christie, Nuclear Instruments and Methods 121, 139-149 (1974) , hence the angle at which the charged particles were emitted from the analysis source can be determined.

Electronic processing of the charges arriving on the detector plate strip can be of usual form as illustrate in Fig. 5 of the drawings. The pulses are amplified and fe to timing single channel analysers. One pulse, the stop pulse, is delayed by the total transit time of the detector strip (~lu sec) so that it always arrives at the Time to Digital Converter after the start pulse. Each digitized time difference is. then a register address in a histogram memory of the control computer (LeCroy 3500) and causes that register to be incremented by 1.

The histogram memory will thus contain counts as a function of angle for one particular energy. The complete set of spectra are obtained by stepping the energy of the analyser, usually by varying input lens voltages. Thus at the end of each energy step,, the histogram memory data is reorganised in the data memory to give counts as a function of energy at a particular angle. This process is repeated until satisfactory statistics have been obtained.

A major field of application of the analyser of the present invention is in photoelectron spectroscopy, and the foregoing description is largely based on such an application. As indicated above, however, the analyser can be used in many other forms of electron or ion spectroscopy and the description in terms of the photoelectron technique is for illustrative purposes only. In particular, the foregoing description largely relates to photoelectron spectroscopy using solid samples but it will be understood that the description could equally well be given in terms of the spectroscopy of gaseous samples.

PRACTICAL EMBODIMENT OF THE INVENTION

A practical embodiment of a spectrometer in accordance with the present invention is illustrated in the accompanying drawings, in which:

Fig. 1 is a diagrammatic illustration of the sample region of an angular resolved photoelectron spec¬ trometer, the z axis being perpendicular to the crystal layers of the sample, the plane z = 0 defining the crystal surface.

Fig. 2 is a diagrammatic perspective view of con¬ centrically arranged, substantially hemi-spherical, toroidal electrode sectors and an annular detector plate, a portion of the toroidal electrode sectors being cut-away to show their configuration in cross-section, in^'defining the toro dal-contoured passageway or pathway for deflecting charged part¬ icles (indicated by arrows) from a sample via entrance slits in tubular electrodes (not shown) , and also to show an axial passage defined by the toroidal electrode sectors for accommodating the tubular electrode, with frusto-conical electrodes (not shown) located in the space between the tor¬ oidal electrode sectors and the annular detector plate.

Fig. 3 is a diagrammatic cross-sectional view of the arrangement illustrated in Fig. 2 but showing the tubular electrode located in said axial passag defined by the substantially hemi-spherical toroid electrode sectors and the conical electrodes locat in said space between the toroidal electrode secto and the annular detector plate.

Fig. 4 is a diagrammatic side elevational view of the annular detector plate, the details of which are further illustrated in Fig. 5.

Fig. 5 is a schematic plan view illustrating the annular detector plate, which is of ceramic materi carrying resistive strips on its upper face and is metallized on its lower face, the associated elec- tronics which indicate the arrival of a pulse of charged particles and specify its arrival position on a resistive strip in terms of a digitized time interval measurement, being also shown.

Referring to Fig. 1, in a typical photoelectron experiment the intensity of electron emission as a function electron energy, polar angle of emission β and azimuthal an φ is to be measured. In conventional angle resolved spectr meters, data is acquired for each selected combination of β φ successively, the energy analyser being capable of accept electrons emitted within a range -αo< <+ o. In the spectro meter of the present invention, for a chosen value of φ, al e ec rons w n e a e a - c_ α c p e .into the energy analyser, thereby decreasing the total time required to analyse the emission from a selected crystal surface.

Referring to Fig. 3 of the dr^'awings, the spectrometer will be seen to comprise an angular defining electrode 1; three slitted cylindrical electrodes 2, the electrode 3 providing a primary Herzog slit; the toroidal electrode sectors 4 and 5; electrode 6, which provides a secondary Herzog slit; an focal plane plate 7; a two-element lens system 8 which re¬ focuses the charged particles focus; an electrostatic shield plate 9; a multichannel amplifier plate 10; and an annular detector plate 11 on mounting plate 12.

The sample is supported on the perpendicular axis of the spectrometer in the plane of the entrance slit 1 of the three cylindrical electrodes 2. The three cylindrical elec¬ trodes 2 are located in the axial passage of the substantially hemi-spherical toroidal electrode sectors 4 and 5.

The first electrode 1 defines the field-free region in which the analysis sample sits and its slit defines the^' angular resolution, the three element lens acting as a zoom lens focuses the electrons at the entrance to the analyser and to which a retarding potential is applied which, in the usual operating mode of constant pass energy, is swept to obtain the energy spectra; and the third electrode 3 which is called the Herzog slit, is held at ground potential and correctly terminates the analyser field.

Substantially hemi-spherical toroidal electrode sectors 4 and 5 which define a toroidal-contoured passageway or pathway for deflecting charged particles from the entrance slits by about 130 , have potentials applied, negative to the outer toroidal electrode sector 4 and positive to the inner toroidal electrode sector 5. Given the approximations made in the analysis of the analyser, the voltages for each of the toroidal electrode sectors 4 and 5 with radii r. and r_ are: where V(r, ~) is the voltage on an electrode of radius r, or r-, E is the required pass energy of the analyser- in electron volts, a O is the radius of the main p trath, ⁱ R is th radius of rotation of the generating circle of the toroi^'d, and r, and r~ are the radii of the generating circles of the toroidal electrodes. This creates an electric field whose equi-potentials are approximately concentric circles and within which electrons of an appropriate energy (pass energy) are focused at the frusto-conical α focal plane electrode 7.

The frusto-conical electrodes 8, which are locate in the space between the substantially hemirspheriσal toroi electrodes 4,5 and the multichannel amplifier plate 10, form a two-element lens system which refocuses electrons on the annular detector plate 11 via the multichannel amplifier plate 10. The electrons are refocused as an annulus for counting and analysing by an electronic compute

Referring to Fig. 5, the upper face of ceramic plate 11 has annular strips of resistive material 13, the ends of each strip being terminated by conductive pads 14. The lower face of the ceramic plate is also coated with conductive material to complete the distributed RC delay line.

Following the arrival of a charge pulse from the microchannel amplifier plate 10 at some position on the chosen resistive strip 13, charge flows to both ends of the strip 13 and is amplified by charge sensitive amplifiers 15 The amplified pulses are further shaped by timing single channel analysers 16 so as to be suitable as input pulses to a time to digital converter 18 (LeCroy 4201) . An elec¬ tronic delay 17 of approximately lμ sec is introduced into one signal line to ensure that the pulse appearing at the 'start' input of the time to digital converter in all cases precedes the pulse appearing at the 'stop' input. The output of the time to digital converter is thus a binary coded signal describing the arrival position of the pulse incident on the detector plate. This signal is passed to the histogram data memory of the control computer 19 (LeCroy 3500 system) for further processing and storage.

Claims (1)

1. CLAIMS :

   1. An angular•resolved spectrometer capable of analys the energy of charged particles from an analysis source and simultaneously obtaining spectra with a resolution of -1.0 for a range of angles of emission of the order of 340 ,

   5 characterized by: (I) concentric toroidal electrode sectors which move charged particles with emission angles -α <α<+α any angle β, and a chosen energy, entering at a path midway of the inlet end of an open-ended annular toroidal-contoure passageway formed by said concentric toroidal sectors and b

   _]_0 which an electrical field is arranged to be established, so charged particles with said energy and emission angles (α,β) will be refocused such that those charged particles with differing angles are strongly refocused but those charged particles with differing β angles are only weakly refocuse

   15 thereby to retain the required β angular information at the α focus plane and provide a focus of charged particles into ring form; and (II) a charged particles position-sensitive detector which registers the focus of charged particles in ring form and generates signal pulses determined by the 0 position of arrival of the charged particles on the detecto

   2. A spectrometer according to claim 1, comprising in axial alignment: (A) a charged particles input focusing section embodying a flitted electrode which defines an angl α and refocuses all charged particles from the analysis

   5 source with emission angles - <α<+ and a chosen energy; (E) an energy resolving electrode section embodying said concentric toroidal electrode sectors which move said refocused charged particles with a chosen energy, entering at a path midway of the inlet end of an open-ended annular 0 toroidal-contoured passageway formed by said concentric toroidal sectors and between which an electrical field is arranged to be established, so that charged particles with said energy and emission angles (α,β) will be refocused such that those charged particles with differing α angles 5 are strongly refocused but those charged particles with

   C' differing β angles are only weakly refocused, thereby to re- ' tain the required angular β information at the α focus plane and provide a primary focus of charged particles into ring ^* form; (C) a charged particles output focusing section embodying a slitted electrode which defines the focal plane of the charged particles emitted from the outlet end of said annular toroidal-contoured passageway and provides a secondary focus of charged particles into ring form; and (D) a charged particles registering section embodying said charged particles position-sensitive detector which registers the focus of charged particles in ring form and generates signal pulses determined by the position of arrival of the charged particles on the detector.

   3. A spectrometer according to claim 1 or 2 wherein means which measures differences in arrival times of the signal pulses is provided to determine the angle β at which the charged particles were emitted from said analysis source.

   4. . A spectrometer according to claim 2 or 3 wherein the charged particles input focusing section consists of a set of cylindrical symmetry slitted electrodes which define the angle α and refocus all charged particles from the analy-^rsis source with emission ang³les -αo<α<+αo and a . chosen energy, at a path midway of the inlet end of the annular toroidal-contoured passageway defined by the surfaces of the concentric toroidal sectors, so that by varying voltages applied to this input lens of electrodes, charged particles of various energy can be brought to a focus at the entrance to the toroidal-contoured passageway.

   5. A spectrometer according to any one of claims 2 to 4 wherein the energy resolving electrode section consists of two concentric sectors of toroids spaced-apart so as to form a toroidally-contoured passageway and between which an electrical field is arranged to be established so that charged particles with an energy E entering this field at the mid-point of the passageway and perpendicular to it will move in an almost circular path of radius a equidistant fr each toroidal surface if the electrical potentials on each toroidal sector with radii r., r₂ are:

   where V(r, ,r₂) is the voltage on an electrode of radius r, r~, E is the required pass energy of the analyser in elect

   £. p volts, a is the radius of the main path, R .is the radius o rotation of the generating circle of the toroid, and r, and r₂ are the radii of the generating circles of the toroidal electrodes, whereby charged particles, with the energy E wh deviate in angle (α) from the perpendicular entry path, and for any angle β, where^'α is the angle of deviation in.a pla ^'containing the axis of the spectrometer and β is an angle i a plane perpendicular to this axis, will be refocused by strongly refocusing those charged particles with differing angles but only weakly refocusing charged particles with differing β angles, thereby retaining the required β angula information at the focus plane.

   6. A spectrometer according to any one of claims 2 to

   5 wherein the charged particles output focusing section consists of a set of frusto-conical symmetry slitted electro embodying (i) a second focal plane electrode which, defines t output slit size, and (ii) a two element lens system for accelerating the charged particles to a suitable energy (300 500 V) for transfer of the ring-form focus of charged partic to the position-sensitive detector.

   7. A spectrometer according to any one of claims 2 to

   6 wherein a charged particles microchannel plate is interpos between the charged particles output section and the charged particles position-sensitive detector, said microchannel amp plate .under electrical potential amplifying the charge de¬ livered by each incident charged particle by a factor of ^10 and ejecting the charge for registering on the charged part¬ icles position-sensitive detector.

   ^* u O:-I . ^■ spec rome er accor ng o c a m w ere n e charged particles position-sensitive detector is arranged to be at a higher electrical potential than the exit potential of the microchannel amplifier plate and is disposed below the microchannel amplifier plate to receive the amplified pulses ejected by the microchannel amplifier plate for registering on the detector.

   9. A spectrometer according to any one of claims 1 to 8 wherein the charged particles position-sensitive detector consists of a detector plate embodying one or more charge- detecting strips in the shape of a section of an annulus from whose ends the signal pulses are derived.

   10. A spectrometer according to claim 9 wherein the detector plate consists of a thin ceramic plate coated on the upper side with one or more separate annular resistive strips to which sensing electrodes are attached and on the lower side with a conducting layer which is earthed.

   11. A spectrometer according to any one of claims 1 to 10 in combination with an electronic control computer which processes the signal pulses generated by the charged part¬ icles refocused as an annular pattern on the detector to provide data as a function of energy at a particular angle, but converting the signal pulses into digitized time diff¬ erences which are loaded into the histogram memory of the control computer so as to contain counts as a function of angle for one particular energy, then reorganized in the data memory to give counts as a function of energy at a particular angle.