GB2187038A

GB2187038A - Electron beam apparatus incorporating secondary electron collection

Info

Publication number: GB2187038A
Application number: GB08703409A
Authority: GB
Inventors: Simon Garth
Original assignee: Texas Instruments Ltd
Current assignee: Texas Instruments Ltd
Priority date: 1986-02-20
Filing date: 1987-02-13
Publication date: 1987-08-26
Anticipated expiration: 2007-02-13
Also published as: US4823005A; GB8703409D0; GB8604181D0; GB2187038B

Description

GB 2 187 038 A 1 SPECIFICATION flashes from the scintillator. The voitages

on the first deflection means should be more positive than the Electron beam apparatus most positive voltage of the sample, so thatthe electrons f rom the sample are drawn towardsthe This invention relates to electron beam apparatus 70 deflection means and can subsequently be directed and in particularto such apparatus where secondary to the collector. The further deflection means should electrons from a sample being bombarded by a have voitages which are sufficiently negative to focussed electron beam are collected forthe purpose ref lectthe electrons from the sample which might of studying the properties of the sample giving rise otherwise tend to pass the first deflection means.

to the production of those electrons. 75 The further deflection means may produce a The collection of electrons from a sample is often deflecting electrostatic field which has the opposite performed by drawing those electrons awayfrom effect on the electron beam from that of thefirst the sample in the general direction of the incident deflection means. In practice, thevoltages on the electron beam and then by means of suitably placed further deflection means needed to producethe electrodes deflecting the electrons awayfrom the 80 compensating deflection of the beam can be chosen beam towards some kind of collector, such asJor to bethe correct voitages for reflecting the electrons example, a scintillator and photomultiplier. The from the sample backto the collector.

electrostatic field causing the relatively slow moving In orderthatthe invention may befully understood electronsfrom the sample under examination to be and readily carried into effect itwill now be deflected towards the scintillator has a greatly 85 described with referenceto the accompanying reduced effect on the incident electron beam drawings, of which:

because its constituent electrons aretravelling ata Figure 1 is a diagram of one example of the much higherspeed dueto the high voltages usedto electron beam apparatus according to the invention; generate the beam and usuallythe effects on the Figure2 is a perspective diagram of thefirst and beam are ignored. However, amongstthe effects of 90 further deflection means of the apparatus of Figure this deflection field on the incident beam is a slight 1; and astigmatism which results in whatshould be a Figure3shows a modification of the apparatus of circularspot becoming elongated. This elongation of Figure 1.

thespotarises apparentlyfrom the effectofthe The apparatus shown in Figure 1 has an electron boundarysurface of the electrostatic deflection field 95 gun 1 which produces a beam 2 of electrons used fordeflecting the electronsfrom the sample, focussed on a sample 3 placed on a single-pole which acts as a slightly "cylindrical" lens. Another magnetic lens 4. Secondary and/or electrons come causeforthe distortion of the spot isthe spread of from the sample 3 as a result of the bombardment by speeds of the electrons constituting the incident the incident electron beam 2 and a hypothetical path beam which has the result, analogous to chromatic 100 of one of these electrons is indicated by the line 5.

aberration, that different electrons are deflected by An electron collector is provided and includes an the samefield to a different extent depending on electrically conducting tube 6 directed radially theirspeeds. relative to the construction of the apparatuswhich It is an object of the present invention to provide has a central axis close to which the electron beam 2 improved electron beam apparatus in which the 105 runs. The inner end of the tube 6 has an electrically disadvantage described above is reduced. conductive mesh 7 through which the electrons from According to the present invention there is the sample 3 are drawn, which then pass down the provided electron beam apparatus in which the tube 6 towards a scintillator 8 maintained at a high electron beam is directed to a sample and secondary positive potential such asJor example, 1 M. A electrons from the sample are returned in the 110 photomultiplier 9 produces amplified signaisfrom direction of the beam and then deflected awayfrom the lightf lashes from the scintillator8 resulting from the beam byfirst electrostatic deflection meansto an the impact of the electrons thereon. An electrode 10 electron collector, wherein further electrostatic also of cylindrical form is placed oppositethe deflection means effective on the beam are located electron collector and is maintained at a positive upstream of thefirst deflection means and the 115 potential slightly belowthat of the collector itself, so further electrostatic deflection means are so thatthe electrons are deflected into the collector.

energised asto counteractthe effect on the beam of Both the collector and the electrode 10 should be thefirst deflection means. maintained at potentials which are positivewith Each of the first and further electrostatic deflection respecttothe most positive voltage onthe partofthe means maybe a 4-element electrostatic stigmator in 120 sample3 under examination so that the electrons which oneof the elements of the first means isa fromthesample3 are drawn towards the collector.

laterally extending tube with an electrically Theelectron collector and the electrode 10are conducting grid overitsinnerend nearerthe supported in a structure 11 so asto be insulatedfrom electron beam, and the voltages applied to the it and from each other, the structure 11 being elements of the f irst mea ns are such thatthe 125 arranged to contain an upwardly directed magnetic secondary electrons a re def iected throug h the grid field which is at its most powerfu 1 at the sam pie 3 and into the tube. A sci ntil lator held at a hig h positive weakens prog ressively with distance from the potential, say, 1 Okv, may be located atthe outer end sample 3, so that electrons from the sample 3 are of the tubeto receive the electrons and a contained bythis field and move upwardly in helical photomultiplier maybe placed to receive the light 130paths of increasing pitch. A grid 12 located just 2 GB 2 187 038 A 2 below the electron collector and the electrode 10 of the two stigmators to improve the collection of maybe provided to give a threshold, adjustable by electrons by the electron collector. In this example, the voltage applied to it, for the energy of the the electron col I ectorconsists of a tube 20 of I a rg er electrons from the sample 3which pass the grid and diameter than the other electrodes of the stigmators are detected bythe collector. 70 with a mesh 21 across its innerend. In addition,the Although it is notshown in Figure 1, an extractor tube 20 has its innerend disposed further from the grid maintained ata very high positive potential of axis of the apparatusto provide a largervolumefor 100Ovolts, say, may be located atthe lower end of the deflection of the electronsfrom the sample 3to the structure 11 to accelerate electrons leaving the be displaced from the axis of the apparatus towards sample 3 into the structure 11. A further grid or other 75 the collector bythe electrostatic field. In orderto means for producing an electrostatic field may also maintain the same electrostatic field on the axisthe be providedfor slowing down the accelerated voltage on thetube 20 and mesh 21 should be raised electrons so thatthey approach the grid 12 atspeeds and may bejor example, 100 volts instead of 50volts substantially equal to those atwhichthey leftthe as in the example described above. Moreover,the sample3. 80 electrode 16 is replaced by an L-shaped electrode 22 Asthus far described, the apparatus of Figure 1 is which is cut away at its underside to provide even similarto that already proposed in European Patent more space forthe deflection of electrons towards Application 84307068.1 (0138610) for electron beam the collector. The tube 20, because of its longer apparatus used to examine the potentials of an diameter, may provide sufficient electrostatic integrated circuitwhen in operation and may 85 attraction to collectthe electrons withoutthe need incorporate details disclosed in that application. f6rthe mesh.

The electron collectorandthe electrode 10 maybe Althoughthe invention has been describedwith two elements of a 4-element electrostatic stigmator reference to just two possible constructions forthe ofwhich one ofthe othertwo electrodes is indicated apparatus, other constructions for achieving the at13.Abovethe electron collector and the electrode 90 same resultwill be apparentto those skilled inthe 10, upstream relativeto the direction of the electron art. For example, instead of using a single pole beam 2, there is placed a second electrostatic magnetic lens with the sample standing on the pole stigmator consisting of four elements of which the piece as shown in Figure 1, other arrangements, elements 14,15 and 16 are shown. The elements of magnetic and/or electrostatic, may be used to the second stigmator are maintained at potentials 95 control the paths of the electrons emitted from the which are sufficiently negative to reflect any sample, and where such alternative arrangements electrons from the sample 3 backtowards the are employed the planarthreshold grid 12,which collectortaking into consideration their starting sets the minimum energy for electrons to be passed potentials and the energythey acquire accelerating to the collector, may need to be replaced by a to thefirst stigmator. In addition, the electrodes 14 100 differently shaped grid. A buffer grid, not shown in and 16 are so biassed thatthey deflectthe electron the drawings. may be provided above the threshold beam 2 in the opposite direction to the deflection grid 12, and maintained at a potential that is more produced bythe electron collector and the electrode positive than any part of the sample 3, forthe 10. The voltages on the other electrodes of thetwo purpose of ensuring that any electrons from the stigmators are selected to reduce as far as possible 105 sample passing the threshold grid enterthe control the astigmatism of the spot produced by the electron region of the lower stigmator at a reasonable speed beam resulting from the electrostatic field used to and do not drift slowly into itto be collected by one of collectthe electrons from the sample 3. the three electrodes other than the collector 6.

In a typical example, the electron collector, that is

Claims

to saythe tube 6 and the mesh 7, are maintained at 110 CLAIMS +50 volts

and the electrode 10 at +40 volts. If the voltage of the sample 3 were, say, 10 volts,this 1. Electron beam apparatus in which the electron would mean thatthe potentials of the electrodes 16 beam is directed to a sample and secondary and 14 respectively of the second stigmator should electrons from the sample 35 are returned in the be -30 volts and -20 volts to ensure reflection of the 115 direction of the beam and then deflected awayfrom electrons from the sample 3. The energy of the the beam byfirst electrostatic deflection means to an electron beam
2 itself may be. for example, 1-5 kv so electron collector, wherein further electrostatic thatthe def lection effects of the stigmators will be deflection means effective on the beam are located very much less on the electron beam than on the upstream of the first deflection means and the electrons from the sample
3 which will have an 120 further electrostatic deflection means are so energy corresponding to 40 volts,that is to saysome energised as to counteractthe effect on the beam of 25-125times smaller. This will be the case for any the first deflection means.

electron beam system in which the energy of the 2. Apparatus according to claim 1 in which the electron beam is significantly greaterthan the further electrostatic means is maintained at energy of the secondary electrons. 125 potentials such that secondary electrons from the Figure 2 is a perspective diagram of the electrodes samplewhich mightotherwise passthefirst of the two stigmators shown in the apparatus of electrostatic deflection means and the electron Figure 1 whichwill enable the construction ofthe collectorare reflected to them.

apparatus to be more clearly envisaged. 3. Apparatus according to claim 1 or 2 wherein Figure 3 shows a modification to the construction 130 the first electrostatic deflection means together with 3 GB 2 187 038 A 3 the electron collector and the further electrostatic electrons from the sample to the speeds at which deflection means are 4-element electrostatic they approach the threshold grid.

stigmators, each centred on an axis of apparatus 12. Electron beam apparatus substantially as close to the line of the electron beam and consisting described herein with reference to Figures 1 and 2 of off our cyli ndricai electrodes extending radial ly from 70 the accompanying drawings or modified as the axis in a plane perpendicularto the axis and described herein with orwithout referenceto Figure spaced at 9Tintervals around the axis, 3.

corresponding electrodes of thetwo stigmators being parallel, and the electron collector comprises a conductivetube open at its inner end and forms an electrode of the particular stigmator. Printed for Her Majesty's Stationery Office by Croydon Printing company (UK) Ltd,7187, D8991685.
4. Apparatus according to claim 3 wherein the Published by The Patent Office, 25Southampton Buildings, London, WC2A l AY, electrodes of the stigmators are all of substantially from which copies may be obtained.

the same size.
5. Apparatus according to claim 3 wherein in each stigmatorthree of the electrodes are of substantially the same size, the electron collector tube is of larger diameter than the other electrodes and is spaced further from the axis than the other electrodes, and the electrode of the further electrostatic deflection means which is adjacentto the electron collector tube is cutaway on its side nearestthe electron collector.
6. Apparatus according to claim 3,4 or 5 in which the electron collectortube has a conductive mesh across its inner end.
7. Apparatus according to anyone of claims 3to 6 wherein the electron collector f u rther includes a scintillator located atthe outer end of the tube and maintained at a high positive potential, and a photomultiplier located to receive lightflashes from th e sci nti 11 ato r.
8. Apparatus according to any preceding claim including a threshold grid located between the sample and the first electrostatic deflection means together with the electron collector, at such a position thatthe secondary electrons from the sample approach the threshold grid at speeds bearing a predetermined relationship to the speeds at which they left the sample, a connection being provided to the threshold grid so that its potential could beset so as to allow to pass only secondary electrons having speeds greaterthan a desired value.
9. Apparatus according to claim 8 including a buffer grid between the threshold grid and the first electrostatic deflection means together with the electron collector, the buffer grid being maintained at a potential more positive than any on the sample so asto accelerate electrons from the sample passing the threshold grid.
10. Apparatus according to claim 8 or 9 wherein the sample is placed on the pole of a single magnetic lens and means are provided to constrain the magnetic field to weaken gradually between the sample and the threshold grid so that secondary electrons from the sample follow helical paths of progressively longer pitches towards the threshold grid, so thatthey approach the threshold grid substantially normally.
11. Apparatus according to claim 8,9 or 10 including an extractor grid maintained at a high positive potential near the sample to accelerate rapidiythe electrons from the sample, and other electrostatic means is provided to retard the