GB2177847A

GB2177847A - Electron lithography

Info

Publication number: GB2177847A
Application number: GB08613820A
Authority: GB
Inventors: David Warren Turner
Original assignee: Individual
Current assignee: Individual
Priority date: 1985-06-07
Filing date: 1986-06-06
Publication date: 1987-01-28
Also published as: GB8613820D0; GB8514390D0

Abstract

A method of electron lithography is described, in which electrons emitted from a source (1) in a pattern are directed onto a target (21) in a corresponding pattern by being constrained to follow magnetic lines of force (17) generated at (9), (11) and extending between the source and target. The target pattern may be a reduced form of the source pattern. The lines of force may be parallel to one another for a limited distance from the source and/or target, to eliminate depth of field problems. A detector (29) may detect electrons leaving the target, for use in the correction of registration errors between the source and target. Additional magnetic fields, and electrostatic fields, may be applied by components (23), (25), (27). The source and electrostatic fields may be pulsed. <IMAGE>

Description

SPECIFICATION Electron lithography This invention relates to electron lithography.

In a known method of electron lithography a beam of electrons is directed at a target and the beam is scanned across the target in a desired pattern. One particular application ofsuch a method is in the manufacture of semiconductor devices, the beam being scanned in a desired pattern across a layer of resist material formed on a semiconductorwaferso as to expose selected regions of the wafer. The exposed or unexposed regions of the layer (depending on the resist material) may then be removed and the remaining regions ofthe layer used as a mask against an etching process applied to the wafersurface ora diffusion process applied to the wafer.

In another known method of electron lithography electrons emitted from a source in a pattern are focussed by a combination of electric and magnetic fields onto the target.

In such known techniques, since a beam of electrons focussed in the plane of the target is used, depth of field problems necessarily arise.

It is an object ofthe present invention to provide an alternative method of electron lithography wherein this problem may be eliminated.

According to the present invention there is provided a method of electron lithography wherein electrons emitted from a source ina pattern are directed onto a target in a corresponding pattern by being constrained to follow magnetic lines of force extending between the source and target.

Normallythelinesofforcewill bearrangedto converge in the direction from thesourcetothetarget and the field will be arranged so that the electrons strike thetarget in a pattern comprising a reduced image ofthe pattern in which they are emitted from the source.

Itwill be understood that in a method according ta the invention electrons emitted bythesourcewill follow helical pathsextending in the direction ofthe magnetic lines offorce, and'thatthe diameter ofthe helical paths may easily be arranged to be sufficiently small forthe pattern atthetargetto have high resolution.

In such an arrangement the lines of force are preferablyarrangedto be parallel for a limited distance extending from the targettowards the source and/or a limited distance from the source towards the target. Such an arrangement has the advantage that the positioning ofthetargetand/orsource along the direction of the field is not critical i.e. depth of field problems are eliminated.

In a method according to the invention electrons may be arranged to leave selected regions of the targetfor passage along the lines offorce to the source where they are detected to obtain an indication of the relative positions of the source and targettransverse to the lines of force. Such indication may be used to obtain accurate positioning of the target relative to the source during successive lithographic steps i.e. so that successive lithographic steps may be accurately registered. Electrons may be caused to leave the targetforthis purpose by irradiation of electron emitting material atthetarget,orbyreflectionatthe target of electrons arriving atthetargetfromthe source.

Registration errors between source and target may be corrected in a method according to the invention by application of a small magnetic field directed trans verse to the magnetic field whose lines extend from source to target.

Alternatively such registrations errors may be corrected by application of electrostatic fields erected transverse to the magnetic field whose lines extend from source to target. Where the regsitration errors involve rotation ofthe image, these may be corrected by application of cylindrically symmetric electrostatic fields whose axis of symmetry is directed parallel to the magnetic field whose lines extend from sourceto target.

Control ofthe size ofthe image at the target relative to size ofthe image at the source may be achieved by application of a small magneticfield parallel to the main field extending between the source and target.

This can be particularly useful in correcting for effects such at thermal expansion ofthe source and/orthe target.

Control of the above-mentioned small magnetic fields may be effected in response to the detection at the source of electrons received from the target, as mentioned above, thereby to obtain automatic reg istrationofthe source and target.

Where, as will normally be the case in exposure of resist material in semi-conductor wafer processing, the electrons need to impinge on the target with significantly higher energies than that with which they leave the source, this is suitably achieved by means of an electric field in the region of the target directed parallel to the magneticlinesofforceextending between the target and the source so as to accelerate electrons towards thetarget. In such an arrangement the source is preferably pulsed and the electric field is similarly pulsed so that the electrons travelling between source and target are not subjected to the electric field in any regions where the electric field is not directed parallel to the magnetic field.

The source and the electric field may also advantageously both be pulsed with atime delay chosen to allow only a selected group of electrons emitted from the source to be accelerated. This group will comprise mostly those electrons leaving the source at small inclinationstothe magnetic lines offorce and travelling the shortest path and arriving afterthe shortest time. Other, those most inclined to the magnetic lines offorce, with most degrade the resolution, arrive later and afterthe end ofthe electric field pulse and are thus not accelerated.

The source may also advantageously be pulsed so that electrons may be cleared from the space between the source and target in intervals between the pulses.

In this way the build up of a space charge between the source and target comprising electrons launched from the source at relatively large angles to the magnetic lines offorce atthesource may be avoided.

The invention also provides apparatusforcarrying out a method according to the invention.

One method in accordance with the invention and an apparatus for carrying out the method will now be described by way of example with reference to the accompanying drawings in which: Figure lisa schematic diagram ofthe apparatus; and Figure 2 illustrates diagramatically a modification of the apparatus.

Referring to Figure 1 of the drawings, the apparatus includesasource 1 arrangedtoemitelectrons in a described pattern.

The source comprises a substrate 3 consisting of a material transparentto ultra violet radiation, such as quartz. One main face ofthe source carries a patterned mask layer 5 of a material opaque to ultra violet radiation, such chromium. Overlying the mask layer 5 there is a layer 7 of a material such as caesium iodine which emits electrons on exposureto ultra violet radiation. Alternatively the source 1 may take the form of a caesium coated semi-conductor, such as silicon or gallium arsenide in which atoms of dopants have been inscribed so as to allow emissions of very low energy electrons upon exposure to visible light through the negative electron affinity effect.

The apparatus further includes an annularelectro- magnetic arrangement 9,11 which when energised produces a magnetic field comprising a uniform low field region 13 and a relatively small area uniform high field region 15 between which regionsthe magnetic field converges.

The source 1 is positioned in the low field region 13, perpendicularto the magnetic linesofforce 17, with the layer7 facing the high field region 15, and a source 19 of ultra violet radiation is arranged to illuminate the source 1 from its side remote from the high field region.

In the high field region 1 Sthere is a planartarget 21 disposed perpendicularto the magnetic lines offorce 17.

Adjacent to the target 21, between the target 21 and the source 1 there is a series of spaced annular electrodes 23 which, when connected to appropriate differing potentials, produce an electricfield in a direction parallel to magnetic lines offorce 17 in the high field region 15.

The apparatus further includes an auxiliary annular electromagnet 25 adjacent the source 1 which pro ducesafield parallel tothe main field produced by the main electromagnet arrangement9, 11 and a further electromagnet 27 which produces a field in any desired direction perpendicularto the field produced bythemainelectromagnetarrangement9, 11.

At a suitable location on or adjacent to the substrate 3, such as not to interfere with the desired electron emission pattern ofthe source 1, there is provided a small patterned detector arrrangement 29 responsive to impingement of electrons.

The various components ofthe apparatus are operated underthe control of a microprocessor or computer 30.

In operation ofthe apparatus the source 1 is illuminated bythe UVsource 19sothatthe layer7 emits electrons in a pattern determined by the mask, layer 5. The electrons so emitted in a direction generally parallel to the magnetic lines offorce 17, towards the target, are constrained by the magnetic field to follow the lines of force 17. In accordance with well known principles the electrons follow helical paths, and in a uniform field the maximum helix diameters proportional to E1/2B-1 where E is the energy ofthe electron and B is the magnetic field strength. Ifthefield is not uniform then the diameter of the helical path of an individual electron changes in such a way that the orbit encloses a fixed bundle of field lines.Hence as the electrons travel from the source 1 to the targetthey maintain their relative positions and the electrons therefore impinge on the target in a pattern corresponding to the mask layer 5.

For electrons having a velocity of 0.1 eV and a field in the region of the target 21 of strength 7 tesla the diamater ofthe electron helical paths is 0.3 microns so that an image ofsub-micron resolution is possible at the target 21. If one considers a point on the source 1 this will be mapped by a single field line 17to a point onthetarget2l. Because the electrons return periodically to a given field line the largest transverse excursion of electrons arising from the source point will be equal to the diameterofthe electron paths.

However, since all electrons from a particular point return periodicallytothe samefield line there will be a strong central peak in the electron flux distribution whose dimension is significantly smallerthan that of the electron path diameter.

Image resolution at the targetwill also depend on the launch angles and energies of electrons from the source 1, the source being designed to emit electrons of a limited range of low energies, and a limited angular spread, a concentration of electrons preferablybeing emitted in a direction normal to the surface ofthe source. Where the target 21 is a resist layer on a semiconductor wafer, image resolution will also depend on the contrast (gamma) ofthe resist and electron scattering inthe resist. Such scattering can be minimised by accelerating the electronstothe opti mu m energy required to expose the resist. Furthermore, the magnetic field will modify the longer scattering paths by imposing a Larmorforcecon- straintbetween scattering sites.

The electrodes 23 are used to produce an electric field which accelerates the electrons to the energy with which they are required to strikethetarget21, e.g.

the above-mentioned optimum energy for resist exposure. The electrodes 23 are shaped and positioned so that the equipotential surfaces of the electric field (notshown) are orthogonal tothe magnetic lines offorce 17 in the uniform high field region 15. It will be appreciated, however, that outside the region where the electric field equipotential surfaces are orthogonal to the magneticfield lines the electron pattern would be distorted by the electric field. To avoid this problem the UV source 19 is pulsed and the electric field applied only after each pulse of emitted electrons has entered the region where the electricfield equipotentials and magnetic field lines are orthogonal.

The electric field is also pulsed for such a period and at such atime after each UV source pulse that only those electrons which leave the source at small inclinations to the magnetic field lines 17 and there- fore travel the shortest path to the target and arrive after the shortest time are subjected to the electric field. In this way, those electrons which leave the sourceatrelativelylargeinclinationstothefield line 17, and would therefore most degrade resolution, arrive after the end of the electric field pulse and are not accelerated.

The intervals between pulses of electrons emitted from the source 1 may also be used to prevent build up in the space between source 1 and target 15 of a space charge of electrons emitted from the source at relatively large anglesto the magneticfield lines 17, which electrons will suffer reflection in the converging magnetic field region by the well known magnetic mirror effect. Such electrons may be removed in the intervals between electron pulses. This may be done, forexample, by applying asuitableelectricfield transverse to the magnetic field lines 17to deflect these electrons to a collector (not shown).

The electromagnet 25 is used to correct small errors in the demagnification ofthe image between source 1 and target 21. Small differences in the demagnification between one region ofthe source 1 and another may be corrected by imposing a gradient on the field produced bythe electromagnet25, or a further electromagnet (notshown).

The electromagnet arrangement 27 is used to control the relative position of the image at the target and sourcetherebyto correct for registration errors of the target and source.

Control of the current applied to the electromagnet arrangement 27 may be effected automatically in response to the output ofthe detector arrangement 29. The detector arrangement 29 is a ranged to be responsive to electrons produced at the target 21, which electrons will travel back to the source under guidance of the magnetic lines of force 17 in the same manner as electrons emitted at the source 1 travel to the target 21. To this end the target 21 incorporates a patterned photo-emitter irradiated by a local UV source (not shown). The detector arrangement 29 is designed so that the magnitude and direction of registration errors can be calculated from the output of the detector arrangement.The output of the detector arrangement 29 could of course alternatively be used to correct registration errors by controlling the relative positions of the source f and target 21.

The local UV source may advantageously be pulsed and the output ofthe detectorarrangement 29 measured at an appropriate time interval after pulsing to ensure that only electrons from the local UV source contribute to the detector arrangement output.

Registration errors of source and target may also be corrected using electrons emitted from an auxiliary electron source, for example a photo-emissive region or a thermionic emitter, located adjacent to and fixed with respect to the source 1, the electrons emitted by the auxiliary source travelling to the target where they are reflected by a patterned reflector adjacent to and fixed with respect to the target, and returned to detectors positioned alongside the auxiliary source.

Since electrons emitted from the auxiliary source will otherwise travel from the source to the reflector along a magnetic line offorce and then return along the same line offorce to the source, a tranverse electric field is required to move the electrons across the magnetic lines offorce.

One such arrangement is illustrated in Figure 2. In this arrangement electrons are emitted from an auxiliary source 31 positioned between two detectors 33 and 35. After leaving the source 31 the electrons initially follow the magnetic lines offorce 17 as described above. After a short distance they enter a region of transverse electric field 37 so as to be subject totranslation movementacrossthelinesofforce 17.

After leaving the electric field region 37 the electrons then againfollowthe magnetlineofforce 17 and impinge on a respective one oftwo reflectors 39 and 41. After reflection the electrons are again subject to translation with respect to the magnetic lines of force in the field region 37 before impinging on one orother of the two dectectors 33 and 35 according to the sense ofthe applied electric field, the full and dotted lines in Figure 2 indicating respectively the electron paths for opposite senses of applied electric field.

The sense of the electric field is arranged to oscillate so that the signals produced by the detectors 33 and 35 oscillate, and the source 31, detectors 33 and 35 and reflectors 39 and 41 are positioned so that when the source 1 and target 21 are in the desired registration the returning electrons are distributed in a predetermined manner, e.g. equally, between thetwo detectors 33 and 35. Displacement of the target 21 and source 1 is then indicated by changes in the relative amplitudes and phases of the output signals of the detectors 33 and 35.

The transverse electric field region 37 is convenient ly located nearthe source 31 in the low main field region 73, as shown in Figure 2, but may be positioned eisewhere along the lines of force 17, if desired.

It will be understood thatthetrnnsverse electric field in region 37 is required to be off while electrons from the main source 1 are being directed onto the target 21.Thusa check on registration of target 21 and source 1 will be carried out before exposure of the traget21 toelectronsfromthe source 1, or during intervals in the exposure.

It will be appreciated that registration errors of source andtarget may also be corrected by application of electrostatic fields erected transverse to the lines offorce 17. Registration errors involving rotation ofthe image may thus be corrected by application of cylindrically symmetric electrostatic fields whose axis of symmetry is directed parallel to the lines offorce 17.

Such electrostatic fields will be designed to be of limited extent parallel to the lines offorce 17 so asto exhibit a suitable gradation of radial electric field component variation with radial distance.

Claims

1. A method of electron lithography wherein electrons emitted from a source in a pattern are directed onto a target in a corresponding pattern by being constrained to follow magnetic lines of force extending between the source and target.

2. A method according to Claim 1 in which the lines of forces are arranged to converge in the direction from the source to the target and the field is arranged so that the electrons strike the target in a pattern comprising a reduced image of the pattern in which they are emitted from the course.

3. A method according to either of the preceding claims in which the lines offorce are arranged to be parallel for a limited distance extending from the target towards the source and/or a limited distance from the source towards the target.

4. A method according to anyone ofthe preceding claims in which electrons are arranged to leave selected regions ofthe target for passage along the lines offorceto the source where they are detected to obtain an indication ofthe relative positions ofthe source and targettransverse to the lines of force.

5. Amethod accordingto Claim4inwhichthe indication is used to obtain accurate positioning ofthe target relative to the source during successive lithographic steps.

6. A method according to Claim 4 or Claim 5 in which said electrons are caused to leave the target by irradiation of electron emitting material at the target.

7. A method according to Claim 4 or Claim 5 in which said electrons are caused to leave the target by reflection atthe target of electrons arriving at the target from the source.

8. A method according to anyone ofthe preceding claims in which registration errors between the source and target are corrected by application of a small magnetic field directed transverse to the magnetic field whose lines extendfrom source to target.

9. A method according to anyone of Claims 1 to 7 in which registration errors between source and target are corrected by application of electrostatic fields erected transverse to the magnetic field whose lines extend from sourcetotarget.

10. A method according to Claim 9 in which the registration errors between sourceand target involve rotation ofthe image, and said electrostaticfields are cylindrically symmetric with their axis of symmetry directed parallel to the magneticfieldwhose lines extend from source to target.

11. A method according to anyone of the preceding claims in which control of the size of the size ofthe image at theta rget relative to size ofthe image at the source is achieved by application of a small magnetic field parallel to the main field extending between the course and target.

12. A method according to Claim 11 when dependent on Claim 4 in which control of said small magnetic field parallel to the main field is effected in response to the detection atthesource of electrons received from the target, thereby to obtain automatic registration of the source and target

13. A method according to anyone of the preceeding claims in which an electric field in the region ofthe target is directed parallel to the magnetic lines of force extending between the target and the source so as to accelerate electrons towards the target.

14. A method according to Claim 12 in which the source is pulsed, and the electric field is similarly pulsed sothatthe electrons travelling between source and target are not subjected to the electric field in any regions where the electric field is not directed parallel to the magneticfield.

15. AmethodaccordingtoClaim 14in which the source and electric field are pulsed with a time delay chosen to allow only a selected group of electrons emitted from the source to be accelerated.

16. Amethod according to anyone of the preced- ing claims in which the source is pulsed so that electrons are cleared from the space between the source and target in intervals between the pulses.

17. A method of electron lithography substantially as hereinbefore described with referencetothe accompanying drawings.

18. An apparatus arranged to perform a method of electron lithography according to anyone of the preceding claims.

19.An apparatus arranged to perform a method of electron lithography substantially as hereinbefore described with reference to the accompanying drawings.