CA1149086A

CA1149086A - Variable-spot raster scanning in an electron beam exposure system

Info

Publication number: CA1149086A
Application number: CA000270570A
Authority: CA
Inventors: Michael G.R. Thomson; Robert J. Collier
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1976-02-05
Filing date: 1977-01-27
Publication date: 1983-06-28
Also published as: JPS52103967A; DE2704441A1; FR2340616A1; GB1557924A; JPS5426875B2

Abstract

Collier-Thomson 14-1 VARIABLE-SPOT RASTER SCANNING IN
AN ELECTRON BEAM EXPOSURE SYSTEM

Abstract of the Disclosure For a given resolution or address dimension, the pattern-writing speed of an electron beam exposure system is increased by utilizing a new mode of raster scanning. In the new mode, the writing spot dimensions of the electron beam are varied rapidly during the scan.
In an electron column designed for variable-spot raster scanning, an illuminated aperture is demagnified to form the writing spot. By imaging a first aperture upon a second aperture and rapidly deflecting the image of the first aperture, the portion of the second aperture that is illuminated by the electron beam is altered. In that way, the spot size is selectively varied in a high-speed way during the raster scanning process.

Description

_ckground of the Invention This invention relates to an apparatus and a method for fabricating microminiature devices and, more particularly, to a variable-spot raster scanning technique for use in an electron beam exposure system.
U.S. Patent No. 3,900,737, which issued to R.J.
Collier and D.R. Herriott on August 19, 1975, describes an electron beam exposure system (EBES) that is a practical tool for generating high-quality fine-featured integrated circuit masks. The system is also capable of exposing patterns directly on resist-coated semiconductor wafers. EBES combines continuous transla-tion of the mask or wafer substrate with periodic deflection of the electron beam in a raster-scan mode of operation.
The EBES exposure process requires a beam of electrons emitted from a cathode to be focused to a sub-micron-size spot on an electron-sensitive resist layer. In practice, the diamèter of the spot is also the address dimension of the system. In one particular practical embodi-ment of EBES, the electron beam is focused to a spot 0.5 micrometers (~m) in diameter on the resist layer and is modulated on and off as the spot is successively scanned in raster fashion across a subregion of the layer. Each scan line of the raster has a width of one address dimension and a length of 256 address dimensions. Such a system meets important current needs for moderate-resolution devices (about 2 ~m linewidths with 0.5 ~m resolution) but does not illustrate the ultimate limits of the capabilities of EBES.

`'.-;Y
f~

~9~8~

Various modifications of EBES are possible to adapt it to meet the increasing demand for devices with still smaller features. For example, if it is desired to write 1 ~m minimum features with 0.25 ~m resolution, using the EsES scanning mode described in the aforecited U.S. Patent No. 3,900,737, an electron spot 0.25 ~m in diameter can be employed. However, the penalty that is thereby incurred is that the time required to expose a given area of the resist is increased by a factor of four. For many proposed applications of practical importance this is an economically burdensome penalty which is not acceptable.
Thus, considerable effort has been directed at trying to devise a way in which to decrease the resolution or address dimension of EsES without at the same time increasing the pattern-writing speed of the system. Moreover, it was recognized that such a way, if available, would also increase the pattern-writing speed of EBES in its afore-mentioned moderate or 0.5 ~m resolution mode.
Summary of the_Invention Accordingly, an object of the present invention is an improved raster scanning technique for an electron beam exposure system.
More specifically, an object of this invention is a raster scanning technique in which the writing spot dimensions of the electron beam are varied rapidly during the scan.
Briefly, these and other objects of the present invention are realized in an electron column designed for variable-spot raster scanning. In such a column, an illuminated aperture is demagnified to form the writing spot. By imaging a first aperture upon a second aperture and rapidly deflecting the image of the first aperture, the portion of the second aperture that is illuminated by the electron beam is altered. In that way, the writing spot size normal to the direction of scanning is selectively varied in a high-speed way during the raster scanning process.
In accordance with an aspect of the invention there is provided apparatus for irradiating a surface of a workpiece comprising first means for scanning a charged particle beam relative to said surface to traverse in sequence a plurality of address positions, and second means for varying, in dependence upon a control signal, the extent of the beam at the address positions as said scanning occurs, in which the second means includes a first mask plate having a single beam-transmitting aperture arranged to be illuminated in its entirety by a charged particle beam, a second mask plate having a single beam-transmitting aperture disposed to transmit an image of the beam transmitted through the aperture in the first mask plate only where that image overlies the aperture of the second mask plate, and means for deflecting the image with respect to the aperture in the second mask plate to vary the registration therebetween, in dependence upon the control signal, to vary said extent of the beam, in which a main longitudinal axis of said apparatus extends perpendicular to said first and second mask plates, and in which the cross sections of said apertures as viewed along said axis are non-coincident.
Brief Description of the Drawings A complete understanding of the present invention and of the above and other objects may be gained from a T~ - 3 -consideration of the following detailed description presented hereinbelow in connection with the accompanying drawings, in which:
FIG. 1 is a diagrammatic representation of an electron beam column which is adapted to achieve variable-spot raster scanning;
FIGS. 2 and 3 are specific illustrative depictions of the respective geometries of two apertures that may be included in the FIG. 1 column;
FIGS. 4 through 11 are respective superimposed showings of the apertures of FIGS. 2 and 3 to represent the result of deflecting the image of the FIG. 2 aperture with respect to the FIG. 3 aperture;
FIG. 12 shows a portion of a pattern written by the herein-described variable-spot raster scanning technique using the column of FIG. 1 equipped with the apertures of FIGS. 2 and 3;
FIGS. 13 and 14 are specific illustrative depictions of the respective geometries of two additional apertures that may be included in the FIG. 1 column; and - 3a -FIG. 15 shows a portion of a particular pattern written by the herein-described variable-spot raster scanning technique using the column of FIG. 1 equipped with the specific apertures of FIGS. 13 and 14.
Detailed Description FIG. 1 depicts a specific illustrative litho-graphic apparatus for controllably moving a variable-size electron spot to any designated position on the top surface of an electron-resist layer 10 supported on a substrate 12. In turn, the substrate 12 is mounted on a conventional x-y-movable table 16.
Various positive and negative electron-resist materials suitable for use as the layer 10 are well known in the art. By selectively scanning the electron spot over the surface of the resist layer 10 in a highly ac-curate and high-speed manner, it is possible to make integrated circuit masks or to write directly on a resist-coated silicon wafer to fabricate extremely small and precise low-cost integrated circuits. Some suitable resists for use as the layer 10 are described, for example, in a two-part article by L.E. Thompson entitled "Design of Polymer Resists for Electron Lithography", Solid State Technology, part 1: July 1974, pages 27-30; part 2:
August 1974, pages 41-46.
The apparatus of FIG. 1 may be considered to comprise two main constituents. One is an electron beam column to be described in detail below, which is characterized by highly accurate high-speed deflection and blanking capabilities similar to those exhibited by the column described in U.S. Patent No. 3,801,792, issued April 2, 1974 to L.H. Lin. Additionally, in accordance ~9~

with the principles of the present invention, the depicted column is further characterized by a variable-spot-size scanning capability. This last-mentioned capability in particular will be described in detail below.
The other main constituent of the FIG. 1 apparatus comprises control equipment 14. Illustratively, the equip-ment 14 is of the type described in the aforecited U.S.
Patent No. 3,900,737. The equipment 14 supplies electrical signals to the described column to systematically control scanning and blanking of the electron beam. Moreover, the equipment 14 supplies control signals to the x-y table 16 to mechanically move the work surface 10 during the electron beam scanning operation, as described in the aforecited U.S. Patent No. 3,900,737.
The specific illustrative electron column of FIG. 1 includes an electron source 22 (for example, a tung-sten filament), a grid 24 and an accelerating anode 26 which comprises a cylindrical metal cap with a central aperture in the bottom flat end thereof maintained at ground potential. In -that case the source 22 is maintained at a relatively high negative potential (for example, 10 kilovolts below ground).
The initial trajectories of electrons supplied by the source 22 of FIG. 1 are represented in the drawing by dashed lines. In the vicinity of the aforementioned aperture in the anode 26 these trajectories go through a so-called crossover or source image point 28 which, for example, is 35 ~m in diameter. Thereafter the electron paths successively diverge and converge as the electrons travel downstream along longitudinal axis 30 toward the work surface 10. Successive crossovers or images of the source 8~;

point 28 are represented in FIG. 1 by dots 32, 34, 36 and 38 disposed along the axis 30.
Advantageously, the electron column of FIG. 1 includes coils 40 by means of which the electron traject-ories emanating from the aforedescribed source point 28 may be exactly centered with respect to the longitudinal axis 30. Thereafter the electron beam is directed at a plate 42 which contains a precisely formed aperture 44 therethrough. The beam is designed to uniformly illuminate the full extent of the opening or aperture 44 in the plate 42 and to appear on the immediate downstream side of the plate 42 with a cross-sectional area that corresponds exactly to the configuration of the aperture 44.
A top view of one advantageous geometry for the aperture 44 in the plate 42 of FIG. 1 is shown in FIG. 2.
Illustratively the plate 42 comprises a disc of molybdenum in which the depicted aperture 44 is formed in a high-precision way by, for example, conventional laser machining techniques.
The dashed lines within the opening 44 of FIG. 2 are included simply to facilitate subsequent discussion.
In actuality the opening 44 is a single continuous aperture having straight edges as indicated by the solid straight lines. Illustratively, the single aperture 44 may be regarded as composed of six square segments, each defined by one or more solid straight lines and one or more dashed straight lines. In FIG. 2 the square segments are designated Ml through M6. In one particular illustrative embodiment of the present invention, each of the squares Ml through M6 measures 100 ~m on a side~ When the plate of FIG. 2 is mcunted in th.e FIG. 1 column, the longitudinal axis 30 of the column is perpendicular to and extends through the mid-point of the square M3 shown in FIG. 2.
As stated above, the cross-sectional configuration 5 of the electron beam that passes through the mask plate 42 of FIG. 1 is determined by the geometry of the aperture 44.
In turn, this beam configuration propagates through a conventional electromagnetic lens 46 (for example, an annular coil with iron pole pieces) which forms an image of the aforedescribed aperture 44 on a second mask plate 48 (FIG. 1). Plate 48 contains a precisely formed aperture 52.
Illustratively, the plate 48 is mounted on and forms an integral unit with electromagnetic field lens 49. The lens 49 is not designed to magnify or demagnify the cross-sectional configuration of the electron beam on theimmediate downstream side of the plate 48. But in comb-ination with a next subsequent downstream lens, to be .described later below, the lens 49 serves to maximize the transmission of electrons along the depicted column. With that next lens, the lens 49 is effective to image the crossover point 28 to the center of a beam-limiting aperture 59.
A predetermined quiescent registration of the image of the aperture 44 on the plate 48 of FIG. 1 is assured by including registration coils 51 in the depicted column.
In accordance with the principles of the present invention, the location of the image of the illuminated aperture 44 on the plate 48 of FIG. 1 is selectively controlled in a high-speed way during the time in which the electron beam is being scanned over the work ,~ ..,s., surface 10. This is done by means of deflectors ~0 positioned, for example, as shown in FIG. 1 to move the beam in the x and/or _ directions. Advantageously, the deflectors 50 comprise two pairs of orthogonally disposed electrostatic deflection plates. Electromagnetic deflection coils may be used in place of the electrostatic plates, but this usually leads -to some loss in deflection speed and accuracy. Whether electrostatic or electromagnetic deflection is employed, the deflectors 50 may also be utilized to achieve registration of the image of aperture 44 in the second mask plate 48. This is done by applying a steady-state centering signal to the deflectors 50. In such a case the separate registration coils 51 may, of course, be omitted from the column.
sefore proceeding to describe further the components included in the electron column of FIG. 1, it will be helpful to specify the nature of the mask plate 48 and to illustrate the effect of moving the location of the image of the aperture 44 on the plate 48. A top view of a specific illustrative element suitable for inclusion in the FIG. 1 column as the mask plate 48 is shown in FIG. 3. Aperture 52 in the plate 48 may, for example, have the shape shown in FIG. 3. In one particular embodiment of this invention, the aperture 52 is a laser-machined opening measuring 100 by 300 ~m. Centrally located dot 54 in FIG. 3 indicates the location of the longitudinal axis 30 of FIG. 1 when the plate 48 is mounted in the column of FIG. 1.
Quiescently, the aperture 44 of the mask plate 42 is imaged by the lens 46 of FIG. 1 onto the center of the mask plate 48. Illustratively, the image .
, ~ i projected by the lens 46 onto the plate 48 corresponds exactly in size with the dimensions of the aperture 44.
(If desired, the lens 46 may, of course, be designed to achieve other than a 1:1 projection of the aperture 44.
Or in some cases of practical interest, the lens 46 may be omitted altogether.) By means of the coils 51 the image so projected is precisely centrally registered on the plate 48, as indicated in FIG. 4.
From FIG. 4 it is apparent that only the segments M2, M3 and M6 of the projected image of the illuminated aperture 44 are transmitted through the rectangular aperture 52 in the mask plate 48. Accordingly, for the depicted registration, the electron beam appearing immediately downstream of the plate 48 has a cross-sectional area corresponding exactly to the geometry of the aperture 52. Hence, for the particular illustrative case in which the aperture 52 constitutes an opening 100 ~m wide and 300 ~m high, the cross-section of the electron beam immediately downstream of the plate 48 exhibits the same dimensions.
Subsequently, the cross-sectional area of the electron beam transmitted through the plate 48 of the electron column of FIG. 1 is demagnified. This is done by means of three conventional electromagnetic lenses 54, 56 and 58. In one specific illustrative embodiment of the principles of the present invention, these lenses are designed to achieve an overall demagnification of the beam `propagated therethrough by a factor of 400. More particularly, these lenses are selected to demagnify the aforementioned cross-sectional area of the beam trans-mitted by the mask plate 48 and to image a reduced _ g O~f~

counterpart thereof on the work surface 10. For an overalldemagnification of 400, and for the specific illustrative case in which the cross-section of the beam immediately downstream of the plate 48 measures 100 by 300 ~m, the electron spot imaged on the surface 10 will quiescently be a rectangle 0.25 ~m wide and 0.75 ~m high.
The other elements included in the column of FIG. 1 are conventional in nature and may, for example, be identical to the corresponding parts included in the column described in the aforecited Lin patent. These elements include the beam-limiting aperture 59, electro-static beam blanking plates 60 and 62, an apertured blanking stop plate 64 and electromagnetic deflection coils 65 through 68.
If the beam blanking plates 60 and 62 of FIG. 1 are activated, the electron beam propagating along the axis 30 is deflected to impinge upon a nonapertured portion of the plate 64. In that way the electron beam is blocked during prescribed intervals of time from appearing at the surface 10. If the beam is not so blocked, it is selectively deflected by the coils 65 through 68 to appear at any desired position in a specified sub-area of the work surface 10. Access to other sub-areas of the surface 10 is gained by mechani-cally moving the surface~by means for example, of acomputer-controlled micromanipulator, as described in the aforecited U.S. Patent No. 3,900,737.
As specified above, the rectangular electron spot provided by centrally positioning the image of the aperture 44 on the mask plate 48 is controlled by the 8~

column of FIG. 1 to impinge or no-t onto a specified location of the work surface 10.
A demagnified version of the rectangular area composed of segments M2, M3 and M6 (FIG. 4) is shown in FIG. 12 and designated by reference numeral 70. For ease of conceptualizatlon and discussion, the rectangular electron spot 70 that impinges on the work surface 10 of FIG. 12 is shown divided into three square segments M2', -M3' and M6'. These segments correspond respectively to portions M2, M3 and M6 of FIG. 4. For the particular illustrative case assumed above in which the overall demagnification is 400, each of the segments M2', M3' and M6' measures 0.25 ~m on a side~
Scanning of the beam provided by the electron column of FIG. 1 is represented in FIG. 12 as occurring from right to left in the -x direction along center line 72. Illustratively, 512 equally spaced-apart address positions are assumed to lie along the scanning center line 72. The location of the first several ones of these address positions are indicated in FIG. 12 by arrows designated APl though AP9. At each address position during the linear scan, the electron beam is blanked or not in the manner described above.
Additionally, in accordance with the principles of the present invention, the area of the beam that impinges upon the work surface 10 at each address position is selectively controlled.
As the variable-size electron spot is deflected along a row of the scan field, the spot is intensity modulated by the beam blanking plates 60 and 62 at, for example, a 10 megahertz rate. This modulation rate ~9~

corresponds with a single-address exposure time of 100 nanoseconds, which is compatible with the sensitivities of available electron resist materials.
At the completion of each scan line, the electron beam is rapidly deflected to an initial position to start a next adjacent scan line. In the particular case illustrated in FIG. 12, such deflection or "fly back"
positions the ~eam above address position APl on a new scanning center line 74 which is parallel to and 0.75 ~m above the line 72. In this way, successive lines of a subregion of the work surface 10 are selectively irradiated in a raster-scan fashion. This raster scan mode of operation (without the variable-spot-size feature) is described in detail in the aforecited U.S. Patent No.
3,900,737.
In accordance with the principles of the present invention, the geometry of the electron spot directed at the surface 10 is varied during scanning in a high-speed way in response to control signals applied by the equip-ment 14 (FIG. l) to the deflectors 50. Thus, for example,by applying appropriate deflection potentials to the deflectors 50, the image of the aperture 44 on the mask plate 48 may be moved in the x and/or _ directions. The effect of doing so is illustrated in the next-described set of figures.
FIG. 5 represents the case wherein the projected image of the aperture 44 has been deflected by the array 50 100 ~m in the +y direction and lO0 ~m in the -x direction.
The effect of this relative disposition is that only the illuminated segment M4 of the projected image is transmitted through the opening 52 in the mask plate 48.

In turn, a demagnified version (M4') of the segment M4 is projected onto the wor]c surface 10. This 0.25 by 0 25 ~m version is shown in FIG. 12 as being located at the address position AP2.
In one illustrative mode of operation that is characteristic of the present invention, the deflection signals applied to the deflectors 50 of FIG. 1 are changed (if necessary) while the scanning electron beam is approximately midway between adjacent address positions.
Establishment of the new deflection signals is carried out in a high-speed way. Thus, Eor example, for the case assumed above wherein each single-address exposure time is 100 nanoseconds, the deflection signals required to achieve a specified spot size are, for example, established by the control equipment 14 in about 10 nanoseconds or less.
The geometrical superposition illustrated in FIG. 5 is achieved, for example, by changing the deflection signals applied to the deflectors 50 of FIG. 1 while the scanning electron beam is about midway between the address positions APl and AP2 (FIG. 12). In practice, voltage swings of about 5 to 10 volts applied to electro-static deflection plates are sufficient to change the quiescent representation shown in FIG. 4 to the deflected condition represented in FIG. 5. Such changes can be realized by means of ultra-high-speed amplifiers in about 5 to 10 nanoseconds.
FIG. 6 represents the case wherein the projected image of the aperture 44 has been deflected 100 ~m in the +y direction by the deflectors 50. As a result, illuminated segments M3 and M6 of the projected image are ,,~
".~

transmitted through the opening 52 in the mask plate 48.
In turn, a demagnified version (M3', M6') of the segments M3 and M6 is projected onto the work surface 10. This 0.25 by 0.5 ~m version is shown in FIG. 12 located at the address position AP3.
FIGS. 7 through 10 illustrate other geometrical superpositions that may be achieved in accordance with the principles of the present invention. The demagnified electron spots that respectively correspond to the super-positions depicted in FIGS. 7 through 10 are shown inFIG. 12 at the address positions AP4 through AP7. Each such superposition is achieved in the manner described above by the deflectors 50 deflecting the image of the aperture 44 100 ~m in the x and/or y directions.
As described above, blanking of the electron beam is achieved in the column of FIG. 1 by means of the plates 60 and 62 and the blanking stop plate 64.
Alternatively, blanking may be achieved by activating the deflectors 50 to deflect the projected image of the aperture 44 sufficiently far with respect to the opening 52 in the mask plate 48 that no portion of the projected image overlies the opening 52.
This condition, which is represented in FIG. 11, requires that the deflectors 50 deflect the noted image 200 ~m in the -x direction. (Of course a deflection of 200 ~m in the +x direction would suffice also). In some cases of practical interest, such a relatively large deflection can be achieved by the deflectors 50 sufficiently rapidly so as to make this alternative blanking technique a feasible one. In such cases the elements 60, 62 and 64 can, of course, be omitted from the Fig. 1 column.

9~8~
As mentioned above, the particular confiyurations of the apertures included in the mask plates 42 and 48 of FIGS. 2 and 3 are illustrative only. It is apparent that a variety of other configurations may be selected to achieve the selective superposition that is the basis for the herein-described variable-spot-size technique. Two such other aperture geometries are shown in FIGS. 13 and 14. Mask plate 80 with aperture 82 therethrough (see FIG. 13) may be substituted for the mask plate 42 shown in the column of FIG. 1. And mask plate 84 with aperture 86 therethrough (FIG. 14) may be substituted for the mask plate 48 in FIG. 1. By way of example, each of the segments M7 through M14 of the aperture 82 of FIG. 13 is assumed to measure 100 ~m on a side, and the rectangular aperture 86 of FIG. 14 is assumed to be 100 ~m wide and 400 ~m high.
By deflecting the projected image of the aperture 82 of FIG. 13 with respect to the aperture 86 of FIG. 14, it is possible to form a variety of electron spot sizes on the surface 10. In turn, this capability makes it possible to irradiate high-resolution patterns on the suxface 10 in a high-speed manner.
A portion of a chevron pattern irradiated in accordance with the principles of the present invention and employing the aperture pair 82 and 86 is shown in FIG. 15. Lines 90 and 92 are the actual idealized boundaries of the pattern to be defined on the surface 10.
The depicted grid formed of horizontal and vertical lines spaced 0.25 ~m apart is not actually included on the surface 10 but is shown only to facilitate understanding of Fig. 15. Lines 94 and 96 are scanning center lines corresponding to the lines 72 and 74 of Fig. 12.

Assume that scanning of the surface 10 of - FIG. 15 occurs from right to left, first along the center line 94 and then along the line 96. Those squares of the grid that are irradiated by the electron beam during variable-spot-size raster scanning of the surface are shown shaded. Each such shaded square is designated with a primed symbol to indicate which corresponding demagnified portion of the illuminated aperture 82 actually impinges on the surface 10. The variable-height rectangular spots shown in FIG. 15 at respective address positions are achieved by successively deflecting the image of the aperture 82 by 100 ~m in each of the x and/or y directions.
As specified above, an illustrative electron beam exposure system made in accordance with the principles of the present invention may have an address length of 0.25 ~m and a spot dimension that can, for example, be varied from a square 0.25 ~m on a side to a rectangle 0.25 ~m by l um.
Eor a given writing spot exposure time, such a system can expose areas at a rate about four times as fast as can be achieved with conventional raster scanning of a fixed-size spot 0.25 ~m in diameter. Or a system equipped for variable-spot raster scanning as specified h~rein can write patterns with 0.25 ~m resolution as fast as a conventional exposure system writes patterns with 0.5 ~m resolution.
It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. In accordance with these principles, numerous other arrangements may be devised by those skilled in the art without departing from the spirit and scope of the invention. For example, although primary emphasis herein has been directed to the case of varying the extent of an electron spot in the direction normal to the scanning direc-tion, it is emphasized that the principles of the present invention also encompass the case wherein spot size or location is varied in the direction of scanning. Moreover, these principles are also applicable to radiant beams other than electron beams (for example light, x-ray and ion beams).

Claims

Claims:

1. Apparatus for irradiating a surface of a workpiece comprising first means for scanning a charged particle beam relative to said surface to traverse in sequence a plurality of address positions, and second means for varying, in dependence upon a control signal, the extent of the beam at the address positions as said scanning occurs, in which the second means includes a first mask plate having a single beam-transmitting aperture arranged to be illuminated in its entirety by a charged particle beam, a second mask plate having a single beam-transmitting aperture disposed to transmit an image of the beam trans-mitted through the aperture in the first mask plate only where that image overlies the aperture of the second mask plate, and means for deflecting the image with respect to the aperture in the second mask plate to vary the registration therebetween, in dependence upon the control signal, to vary said extent of the beam, in which a main longitudinal axis of said apparatus extends perpendicular to said first and second mask plates, and in which the cross sections of said apertures as viewed along said axis are non-coincident.

2. Apparatus as claimed in claim 1, in which the configurations of said apertures are dissimilar.

3. Apparatus as claimed in claim 1 or 2, in which at least one of said apertures is non-rectangular.

4. Apparatus as claimed in claim 1, 2 or 3, in which one of said apertures is rectangular and has a cross section which, as viewed along said axis, is coincident with only a constituent part of the cross section of the other aperture.

5. Apparatus as claimed in claim 1, in which each of said address positions corresponds to a plurality of finite areas on said surface each of which is to be selectively irradiated, in which the single beam transmitting aperture of the first mask plate is composed of a predisposed plurality of constituent areas, each corresponding to said finite area, and in which the single beam transmitting aperture of the second mask plate is composed of a pre-disposed plurality of constituent areas, each corresponding to said finite area, the dispositions of the consitutent areas of the first and second mask plates being different whereby deflection of said image with respect to the aperture of the second mask plate causes the finite areas of each address position to be selectively irradiated.

6. Apparatus as claimed in claim 5, in which the plurality of finite areas corresponding to each of said address positions is defined by the aperture in the second mask plate.

7. Apparatus as claimed in claim 1, 2 or 3, in which the first means serves to scan the beam over said surface raster-fashion, and the second means serves to vary said extent of the beam orthogonal to the direction of scanning in dependence upon the control signal.

8. Apparatus as claimed in claim 1, 2 or 3, comprising means for demagnifying the image transmitted through the aperture of the second mask plate and for focusing the latter image onto said surface.

9. Apparatus as claimed in claim 1, 2 or 3, in which the means for deflecting the image is capable of deflecting the image in first and second orthogonal directions (X and Y) in dependence upon the control signal.

10. Apparatus as claimed in claim 1, 2 or 3, comprising means for blanking the beam, in dependence upon a control signal, to prevent it impinging upon said surface.

11. Apparatus as claimed in claim 1, 2 or 3, in which said means for deflecting the image serves to deflect the image sufficiently far, in dependence upon the control signal, to thereby prevent the beam impinging on the surface.