Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J49/00—Particle spectrometers or separator tubes
  • H01J49/44—Energy spectrometers, e.g. alpha-, beta-spectrometers
  • H01J49/46—Static spectrometers
  • H01J49/48—Static spectrometers using electrostatic analysers, e.g. cylindrical sector, Wien filter

Definitions

• 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.
• 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.
• 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 Alk ⁇ 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 ⁇ 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
• 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.
• 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
• 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 ⁇ and refocuses all charged
• 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 ⁇ , but there is sensibly no focusing in terms of the angle
• 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 ⁇ 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.
• 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.
• the spectrometer comprises five major sec ⁇ tions as set out below:
• V(r l,2 ) (2a + ⁇ R) In a o (2r l,2 ⁇ ⁇ R)
• 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 ( ⁇ ) from the perpendicular entry path and for any angl where ⁇ is the angle of deviation in a plane containing the of the spectrometer and ⁇ is an angle in a plane perpendicul to this axis, will be refocused by the toroidal energy resol section.
• cc 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 ⁇ 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 ⁇ , 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• 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.
• a typical photoelectron experiment the intensity of electron emission as a function electron energy, polar angle of emission ⁇ and azimuthal an ⁇ is to be measured.
• data is acquired for each selected combination of ⁇ ⁇ successively, the energy analyser being capable of accept electrons emitted within a range - ⁇ o ⁇ ⁇ + o.
• al e ec rons w n e a e a - c_ ⁇ c p e .into the energy analyser, thereby decreasing the total time required to analyse the emission from a selected crystal surface.
• 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.
• 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
• r, and r ⁇ are the radii of the generating circles of the toroidal electrodes.
• the frusto-conical electrodes 8 which are locate in the space between the substantially hemirspheri ⁇ al 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
• 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.
• 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 l ⁇ 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.

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• 1981
  • 1981-05-08 GB GB8215665A patent/GB2098797B/en not_active Expired
  • 1981-05-08 EP EP19810901208 patent/EP0058154A1/de not_active Withdrawn
  • 1981-05-08 WO PCT/AU1981/000053 patent/WO1981003395A1/en unknown

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