CA1215481A

CA1215481A - Electron lithography mask manufacture

Info

Publication number: CA1215481A
Application number: CA000455071A
Authority: CA
Inventors: Rodney Ward
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1983-05-25
Filing date: 1984-05-24
Publication date: 1986-12-16
Also published as: EP0127919A2; DE3483217D1; US4652762A; GB8314435D0; US4608332A; JPS59227122A; EP0127919A3; EP0127919B1; GB2142158A; JPH065664B2

Abstract

ABSTRACT:

"Electron lithography masks".

A layer of electron sensitive resist (13) on a semiconductor substrate (12) is exposed to a patterned electron beam emitted from an erasable photocathode mask (6,9) in an electron image projector (5). The mask is formed from a transparent, e.g., quartz, plate (6) on which is provided a layer (9) of caesium iodide or other photoemissive material. A photoemissive pattern is defined in layer (9) by selective direct exposure to a beam of photons, electrons or ions preferably in an evacuated carbon-containing environment whereby the photoemission of the exposed areas of the caesium iodide is lowered.
Alternatively using a beam of charged particles with a relatively high current density the exposed parts of the caesium iodide are actually removed by evaporation.
In both cases, the patterned caesium iodide can be removed by rinsing in water and the transparent plate (6) can be reused with -the same or different photoemissive pattern.
(Figure 1).

Description

3L54~

PHB 32.980 l 27.02.1984 "ELECTRON LITHOGRAPHY MASK MANUFACTURE"

This invention relates to a method of litho-graphically defining a pattern in a layer of electron sensitive re~iqt on a substra-te and further relates to an apparatu~ ~or use in such a method.
In the manufacture of high resolution micro-miniature ~olid state devices, particularly semiconductor devices, a layer of electron senqitive resi~t present on a substrate can be axposed to a patterned electron beam projeoted from a photocathode mask using a lithographic tool known as an electron image projector. The principle of the electron image projector is itself well-known and for further information reference is invited, for example, to the paper by J.P. Scott entitled "1:1 Electron Image Projector" which appeared in Solid State Technology, May lS 1977, pages 43 to 47. The main advantages of electron image projection can be summarised as fast exposure times and high resolution capability.
Con~entionally the mask for the electron image projector is made by providing a layer of opaque material, e.g. a layer of chromium 1,000 ang~troms thick, on the front side of a -transparent quartz plate. The opaque layer is patterned using lithographic and etching techniques and then a continuous photoemissive layer, for example a layer of cae~ium iodide 2,000 ang~troms thick, is provided to cover the patterned opaque layer and the e~poYed areas of the plate. Thus, when the reverse side of the mask i~
illuminated with electromagnetic radiation, electrons ars emitted from the photoemissive layer but only from tho~e araas where the patterned opaque layer i3 not pregent.
While the conventional method of manufacturing a photocathode mask produces results which are quite satis-factory it doæs unfortunately involve numerous proce~sing ~k ~Z~48~

PHB 32.980 2 27~02.1984 steps as follows:
(1) the opaque layer is deposited on the transparent plate,

(2) a layer of radiation sensitive resist is provided on the opaque layer,

(3) the resist is selectively exposed,

(4) the resist is developed,

(5) the exposed parts of the opaque layer are removed by etching,

(6) the remaining resist is cleaned from the patterned opaque layer, and finally

(7) the photoemissive layer is provided.
Also, it is noted here that the mask made in -this manner is permanent in the sense that it can be used only with the patterned opaque layer originally defined.
According to a first aspect of the present in-vention there is provided a method of lithographically defining a pa-ttern in a layer of electron sensitive resist on a substrate, in which an electron emiqsive mask com-prising a photoemissive pa-ttern on a transparent plate is illuminated with electromagnetic radiation to cause a patterned beam of electrons to be emitted from the photo~
emissive pattern, and the layer of electron sensitive resist on the substrate is exposed to the patterned electron beam, characterised in that the photoemissive pattern of the mask is defined by selectively exposing a layer of photoemissive material on the transparent plate to radiation which effec-ts a local modification in the electron emissive proportion of the mask.
In this method the photoemissive pattern is defined by direct exposure of the photoemissive layer which has the advantage that the overall number of stages involved in making the mask is reduced. In particular, the steps which are avoided are: (1) providing an opaque layer, (2) providing a resist layer, (3) developing the resist, (4) etching the opaque layer and (5) cleaning off the remaining resistO
Also, by avoiding the developing and etching PHB 32.980 3 27.02.1984 stages there is the further advantage that the photo-emissive pattern can be defined more accurately because it is particularly at these stages in the conventional method where feature si~e variations can easily be intro-duced, the resolution degraded, and the defect count in-creased.
Moreover, the mask of the present method has the advan~age of being ~rasable, i~e. it is not permanent, because the photoemissive pattern can readily be cleaned off the transparent plate and a new pattern defined in a fresh photoemissive laysr on the same plate.
The radiation to which the photoemissive layer is exposed may be a beam of photons or charged particles.
In the case of charged particles, a relatively high dose may be used to remove locally parts of the photoamissive layer, the remaining parts forming the photoemissive pattern of the mask~ Al-ternatively, with a lower dose, a charged particle beam may be used to modify locally the photoemission of the photoemissive layer. In this case, the photoemissive layer remains intact and the un-exposed parts again form the photoemissive pattern. In both cases, the photoemissive layer is exposed to the beam of charged particles in an evacuated environment and, in the latter case, the Applicants have found that it is preferable if the environment contains residues comprising carbon. It is believed that the carbon is instrumental in locally raising the work function of` the exposed parts of the photoemissive layer so that when the mask is illuminated with electromagnetic radiation of an appropriate wavelength the electrons are emitted only from unexposed parts of the photoemissive layer.
According to a further aspect of the present invention there is provided apparatus for use in a method of lithographically defining a pattern in a layer of electron sensitive resist in accordano.e with the first aspect of the invention, characterised in that the apparatus includes a unitary vacuum envelope comprising first and second chambers and an interconnecting barrel ~Z~.5~

PHB 32.980 4 27.~2.1984 extending therebetween, wherein the first chamber con-tains means for #electively expos~g to radiation a layer of photoemissive material on th0 transparent plate -to form the photoemissive pattern on the mask, and the second chamber contains means for projecting a patterned beam o-f electrons from the mask to the layer of electron sensitive resist on the substrate.
This has the advantage that af-ter defining the photoemissive pattern the transparent plate can readily be maintained in a relatively contaminant-free, dry at-mosphere which reduces -the risk of introducing defects in th0 mask. This risk is further reduced if the unitary vacuum envelope further comprises a third chamber contai-ning means for providing the lay0r of photoemissive material on the transparent plate, the interconnecting barrel also extending between the first and third chambers.
Embodiments of the invention will now be des-cribed, by way of example, with reference to the accom-panying drawing, in which:
Figure 1 is a diagrammatic cross-sectional view of apparatus for a method in accordance with the inven-tion, and Figure 2 is a diagrammatic cross-sectional view showing a modified maYk formation stage of another method in accordance with the invention.
The electron lithography apparatus shown in Figure 1 is for use in th0 manufacture of semiconductor devices and comprises a unitary vacuum envelope 1 made of, for example, aluminium and stainless steel. Tha vacuum anvelope I comprises three unpwardy extending chambers 3, 4, 5 and an interconnecting barrel 2 which extends th0rebetween and which has extensions 2a, 2b extending beyond the chambers 3 and 5 respectively. The chamber 2 on the left hand side in Figure 1 is an evaporation chamber, the chamber 4 in the middle is an electron beam column chamber and the chamber 5 on the right hand side is an electron beam image projector chamber 5.
As described in more detail below, it is in ths -PHB 32.980 5 27.02.1984 image projector chamber 5 tha-t an electron sensitive re-sist layer 13 present on a semiconduc-tor substra-te 12 can be exposed to a patterned electron beam projected from a photocathode mask 6,9. The photocathode mask itself is made in the evaporation and electron beam chambers 3 and 4 as follows.
A circular quartz plate 6 11~mm in diameter and 3mm thick is introduced into the vacuum envelope 1 th~ugh a sealable port-entry represented in Figure 1 as a hinged flap 7. The transparent plate 6 is positioned on a movable carriage ~ initially locatedin the barrel extension 2a.
The vacuum envelope 1 is evacuated to a pressure of, for example, 10 5 Torr using a conventional oil pump. Tha plate 6 is then transported on the carriage 8 horizontally along the barrel extension 2a -to the area of the evapora-tion chamber 3 where a photoemissive layer 9 of, for example, caesium iodide 200 angstroms thick is evaporated in known manner onto -the upwardly directed surface of the quartz plate 6 by hea-ting caesium iodide powder 31 contained in a molybdenum boat 30. The plate 6 coated with caesium iodide layer 9 is then transported along the barrel 2 to the area of the electron beam column chamber 4 which contains an electron beam column in -the form of a so-called electron beam pattern generator which in its own right is well-known to those skilled in the art.
For further information about electron beam pattern gene-rators reference is invited, for example, to the paper entitled "Imaging and Deflection Concepts in Electron Beam Lithography" by H~Co Pfeiffer in the Proceedings of the In-ternational Conference on Microlithography, Paris, June 1977, pages 145 to 151.
Basically the pattern generator comprises an electron gun source 10 and three demagnifying lensas l1a, 11b, 11c for focusing the electrons into a beam 20 onto a target plan~. At the target the focussed beam spot may be circular with a gaussian di~tribution or it may be rectangular with fixed or variable size and shapc depen-ding, as is ~ell known, on the precise structure of the 5~

PHB 32.~80 6 27.02.1984 electron optical column. The electron beam 20 is made to scan the caesium iodide layer 9 on the transparent pla-te 6 switching on and off under computer control so as to selectively expose the caesium iodide layer. Using an electron beam which delivers a total dose of, for example J 500/uC~cm and in a carbon-containing evacuated environment it is found that -the photoemissivity of the exposed areas of the caesium iodide is su'bstantially reduced as compared with the unexposed parts. In Figure 1 the shaded parts 9a of the caesium iodide layer repre-sent the parts which are exposed to the electron beam.
The carbon in the evacuated environment is believed to be instrumental in raisingthe work function of the exposed parts 9a of the caasium iodide layer 9 so that when the quartz plate 6 is illuminated with ultra-violet radiation electrons are emitted only from the unexposed parts of` caesium iodide layer 9. Thus the selectively exposed caesium layer on the quartz plate can be used as a photocathode mask in an electron image pro-jector as described in more detail hereinafter.
By virtue of the oil in the vacuum pump hydro-carbon residues find their way automatically into the vacuum chamber. Also7 there will be some carbon and/or carbon-containing molecules present in the evacuated environment as natural constituents of the atmosphere.
Ga,nerally it can be said that there should be at least sufficient carbon in the environment to form a mono layer on the surface of the caesium iodide. In practice, this amount will be far exceeded. If desired, the carbon con-tent can be enhanced by introducing into the envelope 1 a carbon-containing contaminant, but without ~ignificantl,~
raising the overall pressure. Thus, for example 9 carbon dioxide may be introduced at a partial pressure of the _~
order of 10 ' Torr.
It is noted here that instead of u~ing the elec-tron beam to modify locally the photoemission of the caesium iodide layer 9 it may instead be used actually to remove the exposed parts of the caesium iodide layer s~

PHB 32.980 7 27.02.1984 as shown in Figure 2. In this Figure the arrow 200 re-presents the electrun beam. In this case, the dose used may be approximately 1C/cm . The exposed part~ of the caesium iodide layer 9 are removed by evapora-tion leaving the unexposed parts as a photoemissive pattern on the transparent plate 6 which can similarly be used as a photocathode mask in an electron image projector as described below.
Thus, in both cases a photocathode mask is formed by selectively exposing the layer 9 of caesium iodide on the transparent plate 6 to a focussed beam of electrons. In both cases, too, the photoemissive pattern of the mask i9 constituted by the unexposed parts of the caesium iodide layer 9.
The photocathode mask 6,9 thus formed is then transported on the carriage 8 horizon-tally along the barrel Z to the area of the electron image prujector column 5. As mentioned earlier, the principle of electron image projection i8 well known and for more detailed in~
formation reference is invited to the above-mentioned paper by J.P. Scott. Br~fly, i-t can be said that the image projector is a uni-ty magnification tool for copying a pattern from a photocathode mask onto a layer of electron sensitive resist at the surface of a semiconductor sub-strate-In the present method the photocathode masls6,9 is moved, for example using a compressed air-operated gripper mechanism 50, from the carriage 8 in a vertical direction in the chamber 5 towards a semiconductor sub-strate 1Z situated in a hori~ontal plane and having alayer 13 of electron sensitive resist on its downwardly-directed surface. The photocathode mask 6,9 is moved to within approximately 5mm of the resist-coated substrate 12 and is arranged to be parallel thsreto. The carriage

8 is temporarily moved out of the electron image projec-tion chamber 5. The mask iq then illuminated with ultra-violet radiation from a source 14 outside the vacuum en-velope 1. The ultra~violet radiation enters the envelope 1 P~IB 32.980 8 27.02,19~4 through a window 15 and floods the underside of the quartz plate 6 which is transparent -to the ultra-violet radiation causing electrons to be emitted from the photoemissive pattern on the upper side of the plate 6. The semicon-ductor substrate is maintained at a positive potentialwith respect to the photocathode mask in known manner and under the influence of a magnetic field generated by the electromagnetic focussing coils 16,17 a patterned electron beam 19 corresponding to the photoemissive pattern of the mask is projected towards the semiconductor substrate 12 to expose the layer of electron sensitive resist pre-sent thereon. After sufficient time to expose the electron sensitive resist the semiconductor substrate 12 can 'be removed from the chamber 5 for further processing. The same photoca-thode mask can 'be used many times over to expose a similar pa-ttern on other substrates coated with electron sensitive resist. After up to a thousand or more exposures using the same mask -the photoemissive pro-perties of the caesium iodide may have deteriorated to such an extent that a new photoca-thode mask is desirable, The photocathode mask 9,6 is then moved back onto the carriage 8 which transports it horizontally along -the envelope 1 into the barrel extension 2b from where the mask can be removed through a sealable port-exit repre-sented by the hinged flap 18 in Figure 1. The caesiumiodide 9 can then be cleaned from the quartz plate 6 simply by rinsing in water and the quartz plate can 'be reused for another photocathode mask which may have the same or a different photo~missive pattern defined thereon in the sam~ manner as already d~scribed. A different photoemis-sive pattern can be defined by altering the computer pro-gram for controlling the electron beam pattern generator.
It is noted here that in order to avoid charging effects it is often desirable to include a thin conducting layer on the photocathode ma~lc between the quartz plate and the photoemissive pattern. For example, a layer of chromium 200 angstroms ~hick may be deposited in known manner on the upwardly-directed surface of the plate 6 PHB 32.980 9 27.0Z.1984 before introducing the plate 6 into the vacuum envelope 1. For the sake of clarity, this conducting chromium layer, which is so thin as to be optically transparent, is not shown in the drawing.
It is noted here that the description given above is merely exemplary and many modifications are possible within -the scope of the invention. Thus, although the lithograp~y apparatus described above has the various chambers extending vertically from a horizontal barrel, the whole apparatus or its component parts may be oriented differently. For example the evaporation ~hamber and the electron beam column chamber may extend vertically ~rom a horizon-tal in-terconnecting barrel as described above, while the electron image projector chamber may extend in a horizontal plane, transversely to the barrel.
Furthermore, although the lithography apparatus described above is a unitary system, the evaporation chamber, the electron beam pattern generator and the electron image projector may alternat~vely be arranged as separate units. In this case, because of the hygro-scopic nature of caesium iodide, it will be necessary to protect the coated quartz plate in a dry atmosphere when it is being transported between the different chambers.
Also, the photoemissive pattern can be defined in the photoemi~sive layer otherwise than by using an electron beam pattern generator, For example, an electron image projector could be used in which a patterned beam of electrons from a master photocathode mask is projected onto the caesium iodide layer on the quartz plate to make a replica mask which can then be used to expose a layer of electron sensitive resist on a semiconductor subs-trate as described above. Moreover, photon or ion beams may ba used to modify locally the photoemissivity in a manner analogous to that already described for electrons, and ions having higher doses and QnergieS may furthar be used to remove parts of the photoemissive layer again in a manner analagous to that already described for electrons.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of lithographically defining a pattern in a layer of electron sensitive resist on a substrate, in which an electron emissive mask comprising a photoemissive pattern on a transparent plate is illuminated with electro-magnetic radiation to cause a patterned beam of electrons to be emitted from the photoemissive pattern, and the layer of electron sensitive resist on the substrate is exposed to the patterned electron beam characterized in that the photoemissive pattern of the mask is defined by selectively exposing a layer of photoemissive material on the trans-parent plate to radiation which effects a local modifica-tion in the electron emissive properties of the mask.

2. A method as claimed in Claim 1, characterized in that the photoemissive pattern is defined by selectively exposing the layer of photoemissive material to radiation which effects a local modification in the photoemission of said layer.

3. A method as claimed in claim 1, characterized in that the photoemissive pattern is defined by selectively exposing the layer of photoemissive material to radiation which effects a local removal of parts of said layer.

4. A method as claimed in Claim 2, characterized in that the layer of photoemissive material is exposed in an evacuated environment.

5. A method as claimed in Claim 3, characterized in that the layer of photoemissive material is exposed in an evacuated environment.

6. A method as claimed in Claim 4, characterized in that the evacuated environment contains residues comprising carbon.

7. A method as claimed in Claim 6, characterized in that the material of the photoemissive layer is caesium iodide.

8. A method as claimed in Claim 7, characterized in that the radiation to which the photoemissive layer is exposed is a beam of charged particles.

9. A method as claimed in Claim 8, characterized in that the beam of charged particles is a beam of electrons.

10. Apparatus for use in a method of lithographically defining a pattern in a layer of electron sensitive resist as claimed in Claim 9, characterized in that the apparatus includes a unitary vacuum envelope comprising first and second chambers and an interconnecting barrel extending therebetween, wherein the first chamber contains means for selectively exposing to radiation a layer of photoemissive material on the transparent plate to form the photoemissive pattern on the mask, and the second chamber contains means for projecting a patterned beam of electrons from the mask to the layer of electron sensitive resist on the substrate.

11. Apparatus as claimed in Claim 10, characterized in that the unitary vacuum envelope further comprises a third chamber containing means for providing the layer of photoemissive material on the transparent plate, and in that the interconnecting barrel extends between the first and third chambers.

12. Apparatus as claimed in Claim 10 or 11, charac-terized in that means are included for transporting the transparent plate in a first direction along the barrel from one chamber to another.

13. Apparatus as claimed in Claim 10, characterized in that means are included for transporting the transparent plate in the second chamber in a second direction trans-verse to the first direction.

14. Apparatus as claimed in Claim 10, characterized in that means are included for transporting the transparent plate in a first direction along the barrel from one chamber to another and characterized in that means are included for transporting the transparent plate in the second chamber in a second direction transverse to the first direction.