GB2221082A - Spherical mirror energy analyzer for charged-particle beams - Google Patents

Description

2 2 2 11 0 3 SPIERICAL MIRROR ANALYZER FOR ENERGY CHARGED-PARTICLE BEAMS

The invention relates to devices for energy analysis of charged-particle beams and. in particular, to a spherical mirror analyzer for energy of charged-particle beams.

The present invention may be used to best advantage in production of new electron spectrometers for investigating the surface of a solid using photo, X-ray and Auger-electron spectroscopic techniques. particularly raster spectro- scopy.

It is also suitable for other energy analyses of charged-particle beams, for example, in ion mass-spectroscopy of a solid by analyzing the energy of backscattered ions.

At the present time, the development of solid-state physics calls for improvement of its investigation methods. In electron spectroscopy, it is generally necessary to enhance sensitivity and accuracy in measuring the energy and angular distribution of secondary electrons and to increase the microscanning area of a test object by the use of electron spectrometers, in which the energy analyzer serves as an analytical element.

A rapid development of manufacturing methods relating to solid-state microelectronics necessitates,quick and accurate quality control of surface structure of wafers and integrated circuits at different production stages. One nf the techniques used is diffraction X-ray photoelectron spectroscopy based on the utilization of standing X-ray waves. In such a methodg intensity of monochromatic X-radi- ation used and, in effect, density of knocked-out photoelectrons are very small, a factor necessitatin.c. the creation of a novel wide-aperture energy analyzer to reduce the analysis time from tens of hours to minutes.

A conventional electrostatic spherical mirror energy analyzer comprises two spherical electrodes with a potential difference therebetween. A beam of charged particles enters and leaves the deflecting field of the mirror affecting the internal spherical electrode.

There is known an electrostatic spherical mirror cir- cuit (cf. H.Z.03ar-El 11More on the Spherical Condenser as an Analyzerll. Nucl. Instrum. Meth., 1966, V.12, PP-71-76) wherein a point source and image are found at diametrically opposite points in an internal spherical electrode. Such a circuit arrangement is a rare example of an elec-',-ron-optical system characterized by accurate spatial focusina of a charged-particle beam. In the foregoing publication it is shown that, with kinetic energy of particles E and retarding potential V of the electrostatic spherical mirror re- lated as qX = 2 - R 1 (1 E R2 /1/ where q is particle charge and R V R2 are radii of the Anternal and external spherical electrodes, the point source will appear at a diametrically opposite point with25 out spherical aberration.

The known circuit has been unsatisfactory due to the fact that linear dispersion therein-is dependent on the 01 slope of the analyzed charged-particle beam path, while the axial beam path is characterized by zero dispersion. Thus, the aforesaid circuit is unsuitable for use with energy analyzers having high energy resolution.

There is also known a display-type analyzer (cf. Hirochi Daimon "New Display-Type Analyzer for the Energy and the Angular Distribution of Charged Particles". Rev. Sci. Instrum., 1988, V.59, No. 4, p.545) for measuring the energy and angular distribution of photoelectrons emitted at a solid angle of up to 257-. The known analyzer provides for accurate angular focusing of the electrostatic spherical mirror described in the afore-mentinned publication by Sar-El.

In such an analyzer, dispersion is, to a large extent, dependent on the slope of charged-particle paths, a disadvantage substantially limiting energy resolution thereof. Furthermore, inner- sphere windows passing a beam of charged particles under test should meet stringent requirements to obtain good electron-optical characteristics of the analyzer. The use of conventional fine-mesh windows results in defocusing of a widely diverging charged-particle beam due to the refractive and lens effects on the meshes. which, in turn, impairs resolution of the analyze.r.

There is also known a keplertron-type analyzer for the energy of charged-particle beams (cf. R.H.R.itchie, J.S.Cheka, R.D. Birkhnff "The Sp11.1erical Cnndenser as hiah Transmission particle Spectrometerll, Nucl. Instrum. Meth., 1960, V.69 P.157), which is essent ially an electrostatic mirror k f A formed by the field set up between two concentric spherical electrodes. A charged-particle source is shaped as a disk and disposed on the surface of the internal spherical electrode. A charged-particle beam emerging from said source is reflected by the electrostatic spherical field and focused on the slit of the detecting diaphragm, thus forming a circular image whose position is dependent nnthe particle energy due to dispersion characterizing the known energy analyzer. Moreover, the keplertron-type ana lyzer has a fairly great acceptance angle.

However. the aforesaid analyzer has been generally unsatisfactory due to the following factors: 1) inadequate focusing (effected only as a first approximation with respect to the beam initial divergence angle), which appreci- ably limits re-solution thereof; and 2) the need for using as a detecting means a circular slit diaphrapp placed in the mirror field, a disadvantage distorting the working field and impairing detection of charged particles.

An undesirable characteristic of all the known de- vices is a low analysis speed due to a small microscanning area on the surface of the source. which is attributable to the use of a detecting diaphragm placed in front of the detector, its hole passing charged particles escaping only from a limited area of the test object. Making the diaphragm hole larger will increase the background and impair resolution of the analyzer.

It is an object of the present invention to increase resolution under wide-aperture conditions.

1 f Another object of the invention is to increase analysis speed.

The invention resides in providing a spherical mirror analyzer for energy of charged-particle beams, which would 5 ensure accurate angular focusing of a beam under constant-dispersion and wide-aperture conditions and also a larger scanning area on surface under test.

The foregoing and other objects of the invention are attained by that in a spherical mirror analyzer for energy of charged-particle beams comprising a charged-particle beam source, two concentric spherical electrodes connected to a decelerating voltage source, the internal electrode being provided with windows adapted to pass a beam of charg ed particles, and a detecting means, according to the invention, the eharged-particle beam source and the detect ing means are shaped as segments of a sphere arran,red cen trally symmetrically relative to th.e centre nf the sphe rical electrodes, at least one of said segments being dis posed on the surface of the internal spherical electrode.

The invention makes it -oossible to obtain ideal angul ar focusing of a charged-particle beam with dispersion be ing independent of the slope of beam paths, a condition known as isodispersivity of paths. Vnreover, the image of the source representing a randomly shaped segine-!it in any region of the energy spectrum is free from aberrations and is transferred on a scale to a diamet.rically oppo site section of the internal spherical electrode or its gen metric continuation, an advantage providing for authentici ty of image.

k 1 0 Thus, the proposed energy analyzer permits substanti ally increasing resolution at fairly great acceptance angles in conditions providing for an aberration-free image of larger sections of the source.

In one embodiment of the invention. the detecting means comprises a detecting diaphragm representing a seg mental hole in the internal spherical electrode and a de tector arranged behind said hole and formed with a micro channel plate. The proposed analyzer analyzes the energy of charged particles emergin-1from, the surface of the test nbject which is, in the general case, a spherical segment arranged on the geometric continuation of the inner sphere with all paths being isodispersive and the source image having no aberrations. With high resolution essentially un- CD charged, it is, thus, possible to increase by several tires the working area of the source and the solid angle containing the charge d-part icle beam under test. If the detecting diaphragm arranged centrally 1 symmetrically with respect to the source represents a segmental hole in the inner sphere and accommodates a microchannel plate having its surface aligned with the surface of the inner sphere, a larger area of the test object will be subjected to microscanning in the Drimary-beam scanning mode with high resolution essentially unchanged.

In another embodiment of the invention. the detecting means is like-,. jise aligned with the di-aphra.m, hnle and comprises a position-sensitive detector. The use of said position-sensitive detector is necessary when the surface Qf the test object may not be scanned with a narrnw-focused beam, for example, with excitation derived from an XC-ray tube. In this case X-rays are incident upon the entire area of the segmental section of the source. The utilized position-sensitive detector comprising a microchannel plate furnishes information on position of the sought-for quantity (element) at a predetermined point of the test object owing to authenticity of image, a feature increasing speed in making a microscanning analysis of the surface under test.

In both embodiments of the proposed spherical mirror analyzer for energy of charged-particle beams, windows on the surface of the internal electrnde comprise a plurality of longitudinal slots disposed in meridional planes con- verging on the axis of symmetry passing through centres of the spherical segments of the source and the images. Such a system of wi.-idnws essentially precludes the refractive effect of charged-particle beams in the meridinnal planes on entry to (or exit from) the retarding field, an advant- age substantially imprnving angular fncusing nf beams and increasint, resolution of t-he spherical mirror energy analyzer.

Other objects and advantages of the invention will become evident from the detailed description of the prefer- red embodiments thereof, taken in conjunction with the accompanying drawings, wherein:

Fig. 1 shows diagrammatically a spherical mirror analyzer for energy of charged-particle beams, according to the invention; i 1 - a - Fig. 2 shows a path of a charge d-part icle beam in internal reflection from an electrostatic Spherical m-i-rrnry according to the invention; Fig. 3 illustrates the property of image authenticity 5 in the spherical mirror, according to the invention., Fig. 4 shows an embodiment of the spherical mirre-r analyzer for energy of charged-particle beams, according to the -Invention; and Fig. 5 shows diagrammatically an alternative embodi- ment of the spherical mirror energy analyzer, according to the invention.

The proposed spherical mirror analyzer fer enero. of y charge d-particle beams (Fig. 1) comprises two spherical enncentric electrodes, more specifically, an internal elec- trode 1 having a radius R 1 and provided with windows 2 suited to pass particles and anexternal electrode 3 having a radius "2 and receiving a decelerating voltage from a corresponding source (not shown in the drawing), a charged-particle beam source 4, a detecting means 5 cnmDosed of a diaphragm 51 and a detector 6, and an exciting radiation generator 7 (an electron gun or X-ray or ion unit). The charge d-part icle beam source 4 representing a spherical segment with its centre at point A is disposed on the surface of the inner sphere of the electrode 1 or on its geo- metric continuation, as shown in Fig. 1. The diaphragm 51 representing a segpental hole in the internal spherical electrode 1 with centre B is arranged centrally s.,vme trically with respect to the source 4. The charged-particle detector 6 is placed behind the diaphragm 51.

4 01 The proposed spherical mirror energy analyzer may use reversed circuitry with the source 4 placed at Point B in the internal spherical electrode 1 of the enera, analyzer, ly while the detecting diaphragm 5' is arranged diametrically opposite the source 4 on the geometric continuation nf the spherical surface of the internal electrode 1. In the latter case the detector 6 is also placed beyond the internal spherical electrode 1. With such a circuit arrangement, the parameters of the energy analyzer do not essentially change.

In the general case the source 4 and a hole in tlhe detecting diaphragm 51 are randomly shaped spherical segments, - more specifically, round or fnur-angled spherical segments or narrow strips.

In its operition the proposed energy analyzer uses the following two properties of a spherical mirror with ideal focusing: energy isodispersivity of beam paths and authenticity of image. For illustration, consider the electrnn-nptical relationships characterizing passage of charged-particle beams through an electrostatic spherical mirror with ideal angular focusing. Fig. 2 shows the path nf a charged-particle bear,, 'ú.n internal reflection from the electrostatic spherical mirror. Suppose that the point se)urce and the image are disposed on the axis of symmetry at points A and B. Referring to tile drawing angular enordinates xl. X29'\/' an d o 1 denote the position of entry and exit points on the surface of the internal spherical electrode 1 and the path slope with respect to the axis of symmetry in the internal spherical electrode 1. Proceeding from the laws of cnnserv- JP ation of energy and momentum we get X 1 - o', = o,-/ 1 - X 2 and whe re 2 2 /), S = qv S - (25 - 1) sin (X j/4/ 2 where S - reflection perimeter of electrostatic spherical mirror; q and E - charge and kinetic energy of particles; and V - deflection potential between electrodes 1 and 3 of mirror.

Turning again to Fig. 2 it is apparent that the distances from the source and the image to the sphere centre are, respectively, /2/ X sin 2 (x 1 9) sin7 /3/ 11 = and sin(xl - I>/-) sin o,-' sin(x 12 = sin 0 /5/ /A/ Linear values are expressed in fractions of the radius of the internal spherical electrode 1. '.Vith ideal angular focusing S = 1. From equation /3/ we get X = 2 (x 1 - e/-) /7/ - Substituting /7/ into /2/ we obtain c= x/ 1 and 1 1 = 12 = 1 by reference to /5/ and /6/, At S = 1 both branches d' IF of a random path beyond the spherical field are inclined at the same angle o relative to the axis of symmetry, while the source A and the image B are symmetric about the sphere centre. Consider that energy dispersion of the sphe- rical mirror is taken for conditions characterized by ideal angular focusing. The energy dispersion D will then characterize the image shift in the energy analyzer when the beam energy changes. So, we have E r,-) /8/ In the given case, dispersion along the axis of symmetry will be differentiated /6/ with respect to energy, that is, D = 21 2 Cos - m Vf 7 - 7ls i nc /C /9/ From equation /9/ it follows that at 1 ---' 1 t-'.-.e dispersion is, to a large measure, dependent on the path slope ->.

However, as 1 increases, the functinn D(c) becomes es sentially a step function. At 1 ----> 1 the) curve turns into a step of double height. So, if the source 4 and its image are found on the surface of the internal SDherical electrode 1, then with ideal angular focusing (at S = 1) the energy dispersion of the spherical mirror is maximum (equal to two radii of the internal spherical electrode 1) and independent of the angle at which particles escape from the source 4 (property known as isodispersivity of paths in a spherical mirror). 7Nith ideal angular focusing, the increment factor is unity. Therefore, the image of the source 4 representing a randomly shaped spherical segment disposed on the surface of the internal spher- Ir 4 Ar ical electrode 1 or on its geometric continuation is trans ferred centrally symmetrically on a one-to-one scale to a diametrically opposite section of the electrode 1 (authen ticity of image).

Fig. 3 illustrates the property of image authenticity in the spherical mirror, according to the invention. Turn ing to the drawing there occurs convergence of spatial beams of charged particles whose energy corresponds to the alignment energy in the case of ideal angular focusing-nf the spherical mirror. As a result, images of the Doint sources a, b and c are formed at centrally symmetric points al, bl and c'. said images being free from aberrations. In this case ab = albl and bc = blcl.

The spherical mirror analyzer for energy of charged15 -particle beams forming the subject of the present invent- cj ion operates in the following manner. The retarding pntential V is applied between the electrodes 1 and 3. The source 4 excited by a primary beam from the --,enerator 7 emits charged particles at different angles c with respect to the axis of symmetry, said particles being passed through the windows 2 into the electrostatic sDherical field. YYhen condition /1/ is satisfied, all the char.al-W%,'& particles emerging from the source 4 are reflected by said field and, regardless of the slope or-, get into the seg- mental hole in the diaphragm 51 whereupon they are detected by the detector 6. Fig. 1 shows only one beam of a plurality of charged-particle beams emitted by the surface of the source 4. Said beam is reflected from the mirror and ideal- Ir ly focused at the centrally symmetric point of the window in the detecting means 5. At a predetermined value of the retarding potential V, particles having the kinetic energy E corresponding to /1/ are focused at the hole in the de- tecting diaphragm 51. Owing to linear energy dispersion amounting to 2R 1 for all paths regardless of the slope c/particles having other energy values are not focused at the hole in the diaphragm 51 and are dispersed so-that only a small quantity thereof gets into the hole in the diaphragm 51. By changing the retarding pntential V it is possible to record in respective sections the entire kinetic energy spectrum in the analyzed charged-particle beam.

Operation of the proposed energy analyzer will nn-.v be considered, by way of example, by reference to the circuit shown in Fig. 4. The datect- or 6 is a microchannel plate representing a spherical segment fitted in the hole in the diaphragm 51 so that its outer surface is ali-,ned with the surface of the internal spherical electrode 1. In the illustred embodiment, use is made of such properties as iso- dispersivity of beam paths and authenticity of image in a mirror characterized by ideal focusing.

Operation of the proposed energy analyzer is essentially as follows.

1. Scanning mode. As shown in Fig. Al, an exciting mic- roprobe representing a thin electron, ion or photon beam from the generator 7 scans line y line the entire segmental surface of the source 4. Secondary electrons reach the spherical mirror adJusted for ideal angular focusing of electrons i -01 having preset kinetic energy corresponding, for example, to a certain Auger transition in atoms of the sought-fnr chemical element. Owing to the property of image authenticit - -> Y characterizing the spherical mirror, the focused beam of:se- condary electrins having preset kinetic energy scans the surface of the microchannel plate 6 in synchronism with the microprobe moving on the surface of the segment 4. The centrally synmetric image will reproduce the scanned area if the source 4. At each moment a record is made on the micro- channel plate 6 only locally, that is, in a section on which the secondary-electron beam is focused at a given mo ment. The signal taken from the microchannel plate 6 is amplified and used to modulate the intensity of the elec tron beam in a video signal processing arrangement wherein the beam scanning is effected in synchronism with,the micro probe moving on the surface of the segment, 4. The image formed on the screen of the video signal processing arrange ment shows distribution of sources if secondary electrons with Dreset energy over the scanned area of the se!-,ment 4.

2. Static mode. The generator 7 emits a wide beam of radiation (electrons, ions, photons) exciting secondary electrons, said beam being used to illuminate the segment 4 in a uniform manner. The spherical mirror is adjasted for ideal focusing of secondary electrons with preset kinetic energy (for example, with energy of a certain Auger transition or differential energy of excitin.o. quantum and binding energy of internal atomic level). The secondary-electron beams escaping from different sections of the sep jnent 4 are ( A f 4 F - 15 focused on the surface of the position-sensitive detector, thus forming a centrally symmetric image of the exposed area of the segment 4. The background signal originated due to unfocused secondary electrons having other energy values and the legitimate signal are applied to the amplifier and collector systems of the recording means wherein the noise is suppressed and position data are obtained to characterize distribution of sources of secondary electrons with preset energy in the exposed area of the sePnent 4.

In both modes. owing to the property of image authenti- city characterizing the spherical mirror. the area to be exposed is not limited in the proposed energy analyzer. It way constitute a considerable Dortion of the surface of the internal spherical electrode 1 exceeding by two orders of magnitude the scanned area in the known electron spectrometers. 'dhen the diameter of the internal spherical electrode 1 is large enough relative to the size of the snurce 4, said source may be disk-shaped and the surface of the detector 6 may be also flat. In this case focusing quality and dispersion value will not be essentially affected.

Fig. 5 shows diagrammatically an alternative embndiment of the spherical mirror energy analyzer wherein aperture windows 2 in the internal spherical electrode 1 comprise a plurality of longitudinal slots arranged in meridi- onal planes converging on the axis of symmetry.

The energy analyzer also comprises a system of circular conductors which are at electric potential to protect the working regions of the field. The illustrated energy k analyzer operates in a manner similar to that described above. The marginal field of a separate slot is a two-dimensional field independent of the polar angle. Therefore, in passage through the aperture window 2, the b@am paths are not refracted in meridional planes and the focusing of a widely diverging beam is not distorted. The slots shnuld be narrow ennuz,h to obtain minimal field disturbances on the surface of the internal spherical electrode 1. The gaps between the slots should be small so that transparence 10 of the aperture window 2 is not impaired.

The present invention may be used in production of' new electron spectrometers for investigating the surface of a solid using photo, X-ray and Auger-electron spectroscopic techniques, particularly raster spect_rnscopy.

1 i i

Claims (5)

1. A spherical mirror analyzer for energy of charg.,e d- -particle beams, comprising a charg-p-d-particle beam source, two concentric spherical electrodes connected to a decele rating voltage source, the internal electrodes having win dows adapted to pass a charged-particle beam, and a detect ing means, in which, according to the invention, the charg ed-particle beam source and the detecting means are shaped as segments of a sphere arranged centrally symmetrical ly, at least one of said segments being disposed on the surface of the internal spherical electrode.

2. A spherical mirror analyzer for energy of charged-particle beams as claimed in Claim 1, wherein the detecting means is formed with a detecting diaphragm representing a segment.al hole in the internal spherical electrnde and with a detector comprising a microchannel plate. 3. A spherical mirror analyzer for energy of charged-particle beams as claimed in Claim 1, wherein the detectin- means is essentially a position-sensitive detector. 20 4. A spherical mirror analyzer for energy of charged-particle beams as claimed in Claim 1 or 2 or 3, wherein the windows on the surface of the internal electrode comprise a plurality of longitudinal slots arranged in meridi onal planes converging on the axis of symmetry. 5. A spherical mirror analyzer for energy of charged-particle beams substantially as hereinabove described with reference to, and as shown in the accompanying drawings.

11% Amendments to the claims have been filed as follows 1. A spherical mirror analyzer for energy of chargedparticle beams, comprising a charged-particle beam source, two concentric spherical electrodes connected to a decelerating voltage source, the internal electrodes having windows adapted to pass a charged-particle beam, and a detecting means, in which, according to the invention, the charged- particle beam source and the detecting means are shaped as segments of a sphere arranged centrally symmetrically relative to the centre of the spherical electrodes, at least one of said segments being disposed on the surface of the internal spherical electrode.

2. A spherical mirror analyzer for energy of chargedparticle beams as claimed in claim 1, wherein the detec ting means is formed with a detecting diaphragm representing a segmental hole in the internal spherical electrode. and with a detector comprising a microchannel plate.

3. A spherical mirror analyzer for energy of chargedparticle beams as claimed in claim 1, wherein the detecting means is essentially a positionsensitive detector.

4. A spherical mirror analyzer for energy of chargedparticle beams as claimed in claim 1 or 2 or 3, wherein the windows on the surface of the internal electrode com--prise a plurality of longitudinal slots arranged in meridional planes converging on the axis of symmetry.

5. A spherical mirror analyzer for energy of chargedparticle beams substantially as hereinabove described with reference to, and as shown in the accompanying drawings.

Published 1990 at The Patent Office, State House, 66.71 High Holborn, London WC1R 4TP. Further copies maybe obtainedfrom The PatentOffice. Sales Branch. St Mary Cray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent, Con. L'87